DISCLAIMER: THIS GUIDE WILL DETAIL A LOT OF MECHANICS YOU MAY NOT WANT TO KNOW ABOUT AND COULD RUIN THE GAME FOR YOU. READ AT OWN RISK.
tl;dr: at the bottom.

DISCLAIMER2: The game has changed a lot through expansions and patches, and a lot of the numbers changed too. They're even dependant on which expansions you have running, so I'm not even going to bother typing up all possibilities. Some existing mechanics have changed, and some new ones have been added. I've tried to get the guide up-to-date but tell me if i've missed something!

Hello. I'm me. My name is to the right. You may know me from such FAQs as "Worms: Forts Under Siege", or...well that was about it. I post a lot in comments though. Minimally active on the Civ Reddit.
This guide was made because I also made a guide to beat Deity in a less cheesey way, which was a culture victory. Explaining the mechanics in one guide and my playthrough in another made sense.

A "Culture Victory" is horribly named. You don't win with Culture. You win with Tourism. You defend with culture. But defending with bullets is often better. A Culture Victory requires you to use pretty much all features of the game, so disrupting in half the areas is usually enough.

The civilopedia calls it the most difficult one. Yet people often complain about it being easy, because they get an 'accidental' Culture Victory while trying for a different kind of Victory. This is because they play against AI, where you are pretty much better than every other player, in every area. How many cities do you have? How many city-states are you suzerain of? How far ahead are you in research/civics? How many wonders has the AI built, that you wanted to have too? That's what I thought.
So this is for the people that either tune the game to actually be hard, or for those playing multiplayer.

A Culture Victory is won when a Civ has more visiting tourists than any other civ has domestic tourists.

Your domestic tourists is simply a linear function using your lifetime culture. Getting a lot of culture/turn lategame will thus mean you still have less domestic tourists than someone that focused on it all game.

This is also why it's painfully easy to win a Culture Victory against AI: they don't usually have a lot of Culture. Nor do they have the common sense to counterspy the places they keep their Great Works, which they still earn. This has gotten better in the Spring 2018 update though.

If two civilizations are already going the Culture/Tourism route, you don't necessarily need to do that as well to 'defend': the other civ is probably going to have more domestic tourists than you, and only the highest opponent counts. If you check the World Rankings > Culture Victory tab, you will see one civ having the most domestic tourists, and all the other civs need to get 1 more visiting tourists than that. The civ in the lead needs to get 1 more tourist than the second-most cultural civ.

The other way to defend is instead to pummel someone that's close to a Culture Victory. Wiping him out works, but you need only take/pillage wherever he is generating Tourism. The AI often give away Great Works in peace deals, so you may not even need to capture cities. Even disturbing trade Routes can help. Just look at the "offense" section and hamper everything in there, mkay?

A Culture Victory is won when a Civ has more visiting tourists than any other civ has domestic tourists.

Every turn, all your sources of Tourism add that number, modified, to every other civ's lifetime Tourism. The visiting tourists from that civ are a linear function of the lifetime Tourism you have built up towards it. When looking through the Tourism Lens, you can see the amount of tourists "in that tile", but it just distributes your visiting tourists over your tiles. The tourists aren't actually there. Losing that tile, pillaging it, or even nuking it, does not kill the tourists. It will just stop generating Tourism, but the lifetime Tourism built up is not lost.

It's easier to win a Culture Victory the more opponents there are. After all, with 3 opponents every opponent needs to contribute a third of the visiting tourists you need to win on average, while with 11 opponents you need much less Tourism for the same number of visiting tourists.

Sources of Tourism are:

• Great works of Art, Music, and Writing
  
• Artifacts
  
• Relics
  
• Holy Cities
  
• Wonders
  
• Walls
  
• National Parks
  
• Seaside Resorts
  
• Buildings

Tourism can be boosted by source, and according to which civ they influence.

Relics, Holy Cities, Religious Works of Art (and some wonders, I assume) are religious sources of Tourism: when a civilization has the Civic "The Enlightenment"(Renaissance Era), the Tourism from those sources is halved.
This can be Counteracted with the Christo Redentor(giant jesus on top of a hill[weebls-stuff.com]) Wonder, which also doubles the Tourism from Seaside Resorts.
Policies that affect Tourism are Heritage Tourism(double the Tourism from Artifacts and Relics) and Satellite Broadcasts(triple the Tourism from Great Works of Music).
Most important is the technology Computers, which doubles all Tourism. Just. All. Tourism.

Other factors include whether you have the same Government(up to -30% depending on your/their government), whether you have Open Borders with that civ(+25% if you do), and whether you have an active Trade Route to them(+25%). The trade route bonus can be boosted by the Online Communities policy(unlocked by Social Media, in the Information Era)for another +50%. Some Great People also give a 25% bonus when a Trade Route is present. Being at war with a civ does not impose a penalty, but does break your Open Borders/Trade Routes with that civ.
All factors stack multiplicatively, and can be checked in the Culture Victory tab when hovering over a civ's visiting tourists.

Works of Art can be created by your Great People, Artifacts are dug up from Antiquity Sites(or dived up? from Shipwrecks). Themed Museums can double the Culture and Tourism of the works in it; Art musea are themed when all three Works are the same type of Art(sculpture, landscape, portrait, religious) in all three slots, and from different artists. Multiple Works from the same artist get penalties instead. Spread works from the same artist around different musea, or trade Works with other civs. There is no such penalty for Archaeological musea, but they require three artifacts both from the same era and with different origins to be themed. When you dig up an Artifact you are prompted to decide what origin it has, choosing between a civ/city state based on the location of nearby cities, or barbarians. Sometimes this prompt is not given and the origin is decided for you. Keep track outside of Civ6 which artifacts you already have, because you can't check your current artifacts during this prompt.

Relics are not as common, mostly through being the Suzerain of Kandy or by having Apostles die while they have the Martyr promotion. The Mont St. Michel Wonder gives every Apostle the Martyr promotion for free, while being the Suzerain of Yeheran allows Apostles to pick any promotion(they get one when trained, but are limited to 1-3 random promotions normally), so you can force them to be martyrs.

Once you have the Conservation civic, a lot changes. Walls will give Tourism, but you can't build them any more after you've researched Steel, so build them before that. You can purchase Naturalists with faith, which are single-use civilians to make a National park, which generates Tourism based on Appeal. After researching Radio you can build Seaside Resorts with builders, which also use Appeal. More on those later.

Entertainment also attracts tourists, and buildings from your Entertainment Complex and Water Park, and even the capitalist Shopping Mall generate Tourism. While the amounts are small, you can build them in each of your cities, and they might just let you beat a rival Tourism farmer.

Some leaders also influence Tourism. The Queen Mother of France doubles all Tourism from wonders, and Theodore Roosevelt has the Film Studio building(replaces the Broadcast Centre), which makes all Tourism from that city twice as strong to other civs in the Modern Era or beyond.

Last but certainly not least, ALL Tourism is doubled gets a +25% increase once you have the Computers tech. Still quite a lot, but don't sacrifice too much resources rushing it.

A Holy City(where a religion was found) gives +8 Tourism. I only found out because of the lense.
Wonders give 3-9 Tourism each, depending on how old it is. Older wonders give more tourism.

Works of Writing give 4 Culture, 4 Tourism. Writers will create two Works of Writing, and some Generals also create one upon retiring. Tourism is doubled after you research the tech Printing. Two works are held by the Amphitheatre, the very first building in the Theatre Square, and many Wonders can also house them, so housing the works should not be hard.

Works of Art give 3 Culture, 2 Tourism. Artists will create three Works of Art, usually all having the same type. Only the first work of an Artist will give full value in a museum, the others only give +1C/+1T, but duplicates in a non-museum(e.g. Hermitage, Mus.Nat.History) do literally nothing fixed in ??? patch. Spread your works from the same artist through your venues to prevent this. A Themed Museum gives +18C/+12T. The Tourism output is doubled by the Heritage Tourism policy.

Artifacts give 3 Culture, 3 Tourism. Duplicate ages/civs do not have the detrimental effect that Works of Art suffer from, but Theming them is a lot harder. There are 5 different eras, and AI have a tendency to pick a Barbarian origin a lot, but a themed Archaeological museum will give you 18 Tourism. Tourism is doubled by the Heritage Tourism policy.

Relics give 4 Faith, 8 Tourism. This is halved to 4 Tourism against civs with The Enlightenment civic(unless Giant Jesus), but that's still 4 Tourism. As much as an entire Artist when crammed into a musem.

Works of Music give 4 Culture, 4 Tourism. Musicians will create two works of Music, but Broadcast Centers hold only one Work of Music, so you need a lot of Theatre Squares to hold them. On the plus side, the competition is a lot smaller. The Bolshoi Theatre, Broadway, and Sydney Opera House have 1/2/3 Music slots though. Tourism is tripled by the Satellite Broadcasts policy.

Buildings grant only small amounts of Tourism, but can be built in each of your cities for a sizable amount of Tourism.
* Walls give 1/2/3(cumulative, 1/3/6) Tourism once you have the Conservation civic. You're unable to build walls after researching Steel, but any walls already built will keep their effect.
* Shopping Malls are built in Neighborhood districts, limit 1 per city. They grant 1 Amenity and 4 Tourism.
* A Stadium grants +2/+5 Tourism if its city has at least 10/20 citizens.
* Ferris Wheels in Water Parks grant 1 Amenity and 2 Tourism.
* An Aquatics Center increases the Tourism generated by Wonders in that city by 2 if they are built on or adjacent to a Coast tile. Far too circumstantial for such a small bonus, don't bother.

Parks give Tourism equal to twice the Appeal of its tiles. Resorts give Tourism(and gold) equal to its Appeal, doubled by the Christo Redentor Wonder.

And of course researching Computers doubles increases all Tourism ever.

In the Modern Era, man discovered Flight. Going to an exotic place to see the sights was suddenly an endeavor that could be done in a week, and later a weekend, or even a day.

While the museums and Wonders of the World have already attracted attention through the ages, Flight causes "Bonus Tourism to all improvements that provide Culture." Even the tiny marvels can now be celebrated by tourists. The Tourism is equal to the culture.

The amount of improvements that provide culture are pretty limited though. None of the basic improvements grant Culture, but Governor Liang's Water Park provides +1 Culture(+3 if he is present) and can be built everywhere. The Rainforest Wonder gives +2 Culture to all Rainforest tiles in the city, which also counts for Flight somehow.

Several civilizations have a unique improvement that gives culture, but most of those have pretty strict placement requirements, like not building them adjacent to one another:

• The unique tile improvement of China, the Great Wall, grants +2 Culture for each adjacent Great Wall. A circle around your city center would get 24 Culture and Tourism. Can only be built in a line on the current border of your territory.
  
• The unique tile improvement of Egypt, the Sphinx, grants +1 Culture, +1 Culture if built on Floodplains, +1 Culture after Natural History. Can not be built adjacent to another Sphinx.
  
• The unique tile improvement of France, the Château, grants +1 Culture and bonus Culture if built adjacent to a Wonder. Can only be built adjacent to rivers, usually taken by farms or Commercial districts.
  
• The unique tile improvement of Sumeria, the Ziggurat, grants +1 Culture if adjacent to a River, +1 after Natural History. Cannot be built on Hills.
  
• The unique tile improvement of Persia, the Pairidaeza, grants +1 Culture (+1 Culture with Diplomatic Service) and an additional +1 Culture for each adjacent Holy Site and Theater Square. Cannot be built on Tundra or Snow tiles, or adjacent to another Pairidaeza.

some of the civilizations are excellent for this though,

• The unique tile improvement of Canada, the Ice Hockey Rink, grants +1 Culture for each adjacent tundra/snow tile, for up to +6 Culture, and another +4 Culture if adjacent to a Stadium. Max 1 per city. Canada also has the easiest time making National Parks through its light Cavalry Mountie, which can be produced or bought with money instead of just with Faith, and is usable as a light Cavalry after creating the park.
  
• The unique tile improvement of Sweden, the Open Air Museum, grants +2 Culture for each land type (grass, plains, desert, tundra, snow) in which at least one Swedish city was founded. It updates when founding such cities after the improvement is built. Max 1 per city. Note that having founded the city is enough, so even if the only pieve of snow is in a loyalty-hostile environment, the founding is all it takes.
  
• The unique tile improvement of Mapuche, the Chemamull, grants Culture equal to 75% of the tile's Appeal. Minimum appeal of Breathtaking(4), but no other requirements. Litter the land with Chemamull and a few City Parks/woods.
  
• The Maori have the unique Theater Square building Marae, which gives +1 Culture (counting for Flight) to all the city's tiles with a passable feature or natural wonder. Improved tiles still count, and "features" include floodplains, Volcanic Soil, and even flooded/submerged tiles.

and finally there are some improvements you can get from City-States

• The Military City-State Granada allows your builders to make Alcazar improvements. Alcazar provide +2 Culture, science equal to 50% of the tile's appeal, and count as a Fort. Cannot be built next to another Alcazar.
  
• The Religious City-State La Venta allows your builders to make Colossal Head improvements. Colossal Heads provide +2 Faith, and +0.5/+1 faith for every adjacent Rainforest (upgrades with Humanism). But this faith generates Tourism once Flight is discovered. It's a minor one, as the rainforests prevent woods and even give negative appeal, but if you're already building them it's nice, especially in a city with Chichen Itza.
  
• The Cultural City-State Rapa Nui (Gathering Power expansion) allows your builders to make Moai improvements.
  Moai are great.
  Moai provide +1 Culture, and +0.5/+1 culture for every adjacent Rainforest (upgrades with Medieval Faires(early!)). +2 Culture if on or adjacent to Volcanic Soil. Volcanic Soil requires an eruption to hit the tile, but by the time you have flight all volcanoes should have a nice ring of soil around them. (which begs the question, has this not happened in the thousands of pre-ancient years?).
  Can be built on plains, grassland, or any terrain with Volcanic Soil on it. Cannot be placed adjacent to a Woods or Rainforest tile.
  A volcano can have up to 6 soil around it, and another 12 moai around that also benefit from the +2, and have an adjacency bonus with each other. Even without volcanoes, just carpeting your temperate-climate fields with moai can easily get 4 culture and tourism per tile. The woods and rainforest adjacency is a nuisance, but you can chop the woods/rainforest underneath bonus resources, and get more builders for the chops!

At the start of the Modern Era, you can research(or wait, what verb should I use?) the Conservation civic. It gives you the ability to plant woods with builders on any Tundra, Plains or Grassland tile, and their Hills variants.
Newly planted woods will behave like woods have thus far, giving +1 Appeal to the surrounding tiles, excluding the tile the woods are on. Any already built Neighborhoods, Resorts, or National Parks will adapt to the prettier environment, so planting woods(or removing rainforests/marshes) can boost their use.
Any original woods are turned into old-growth woods, giving +2 Appeal to the surrounding tiles instead of +1. giving +1 appeal to the tile itself. Its only usage is thus in creating a National Park, which can get up to +4 extra tourism if created entirely on old-growth. It's thus hardly worth saving old-growth trees for, as it's better to find a location where you can plant more woods around the park, or that borders more coast/lakes/theater/entertainment/holy/wonders.
Honorable Mentions go to the Eiffel Tower Wonder(+2 Appeal to every tile in your empire) and Teddy Roosevelt(+1 Appeal to every tile in a city with a National Park, including the Park itself).

It also allows you to recruit Naturalists in any city for 1600+200x 800(+100 each buy) Faith. A Naturalist can be consumed to create a National Park on its current tile. A National Park gives +2 Amenities to the city that owns the tiles, +1 Amenity to the 4 closest other cities, and generates Tourism equal to the Appeal of the tiles in it.

The Civilopedia teaches us that a park is four land tiles in a "diamond pattern", they can be mountains or Natural Wonders, as long as at least one of the tiles is passable, all tiles need to be owned by the same city, and each tile has a minimum appeal of Breathtaking(+4) Charming(+2).

• The diamond pattern refers exclusively to vertical diamonds, even though hexes would allow diagonal Parks as well.
  
• While creating a park on Mountains seems logical as they have a fixed appeal of 4, you can get up to 6 in tundra/coast areas easily just by planting woods everywhere, or 7 if there's some old-growth. Natural wonders are great to turn into Parks, as each tile gives +2 Appeal to the surrounding tiles, and most mountain-like wonders are not locked at 4 appeal.
  
• The tiles need to be owned by the same city, but may lie outside of the 3-tile workable tile radius. Plan ahead and don't swap all workable tiles to a new city sometimes, so the park is still possible exactly where you want it. Tiles within a Park can be worked, and the tiles can safely be swapped after the park is built.
  
• No tiles must be improved (removing improvements costs no builder charge), but resources are allowed. Once created, Park tiles cannot be built on, so plant woods on Park tiles before turning it into a Park! can have new woods planted on them, but no other builder action is allowed. You can further increase the appeal by planting woods around the park as well.

The Radio technology, also in the Modern Era, allows builders to create the Seaside Resort improvement on any coastal Desert, Plains and Grassland, without hills, with an Appeal of charming or better(2+). The tile will then generate Tourism equal to its Appeal. An undocumented feature is that it also add the tile's appeal to its gold yield. Plant woods around Seaside Resorts to increase the Tourism and gold generated.

Neither Park nor Seaside Resorts need to be worked to generate Tourism. In fact, it's not even a prerequisite that they are in working range of a city! All that coastal desert you have because your borders keep expanding? Turn them into resorts. Can't plant woods in the desert next to it? Try a Theatre Square, Entertainment Complex or Wonder instead!

The Tourism from Seaside Resorts can also be doubled by the Christo Redentor Wonder. Tourism used to be doubled by the Radio tech but that was way overpowered, as it allowed for this screenshot:

A park with four tiles of +5 appeal gives FORTY Tourism. That's as many as four tens. And that's awesome. But we can do better:

The Rise and Fall expansion introduced Governors. By advancing through the Civics tree, building a Government Plaza and its buildings, and even building specific wonders, you can get Governor Titles. A title can be used to get a new Governor, or by giving a new Title(Promoting) an existing one, each with their own Title tree. A Governor can be assigned to a city, where it has to settle for 3/5 turns until it is Established and its effects start working, except for its loyalty-boosting effect which happens instantly.
With the Gathering Power expansion, existing governors were changed to also work with the new mechanic Power. As I currently have all expansions, YMMV; if it does, leave a comment with what's different for you.
For the purposes of a culture victory, there's just a few important ones.

Pingala, The Educator

• The base promotion gives +15% science and culture to the assigned city, getting you to the right techs and civics faster.
  
• The first two promotions give +1 science/culture to the city per citizen.
  
• The third-tier promotion Curator gives +100% tourism from works of Writing, Art, and Music. While this too is only for the assigned city, if you have a couple of work-containing wonders in the same city it can account for quite a lot of Tourism.

Reyna, The Financier

• The base promotion will make the city acquire new tiles "faster".
  
• Depending on your game version, the Curator promotion may instead belong to Reyna instead
  
• If not, she will instead have Forestry Management: "This city receives +2 Civ6Gold Gold for each unimproved feature. Tiles adjacent to unimproved features receive +1 Appeal in this city."
  This makes National Parks and Seaside Resorts more efficient and more profitable to be worked. Note that even Floodplains, Volcanic Soil, and even flooded/submerged tiles count as a "feature".

Liang, The Surveyor

• The base promotion will make All Builders trained in this city get +1 build charge, and you need a lot for planting woods.
  
• The third-tier promotion Parks and Recreation: "Can construct the City Park improvement in this city. +1 Culture and +2 Appeal. +1 Amenity if adjacent to water. City Parks provide +3 Culture if Liang is in the city."
  While Liang needs to be in the city when the City Park is built, it keeps working after he is moved to another city, and it gives an excellent appeal boost. Note that "adjacent to water" does not necessarily mean a lake or coast, even a river counts.

Can't be bothered with Great People? Steal enemies' Works! Anything in a Theatre Square can be stolen by a Spy in that city. Trade your stealable works against unstealable ones and then steal them back! Trade your Works for thousands of gold(sometimes AI will even give you a whole city) and then steal them back! Spy gets captured, trade a Work for him, then steal it back! The theft never ends. Protect your Works in the Palace or Wonders, or Counterspy the districts(or districts adjacent to them) for a near-100% failure rate.

Don't like sneaking around? Generate thousands of gold and trade for them. Or just trample them in a war and demand the Works in a peace deal. If you take a city with works in it(viewable by hovering the tiles) you also gain control of them; You can even move the Works to your own musea and return the city in the peace deal.

Have a large area able to be national parked? The algorithm will not consider a park if one of its tiles is already part of another possible park, so the optimal location you want might not be highlighted. Improve your tiles so that only your wanted configuration is possible, you can remove them later without spending a charge.

Is your Tourism dominant over all but one civ that decided to farm stupid amounts of culture? Wipe him out!

Culture Victories are pretty complex. You need city planning, faith, culture, science, Great People points, spies, Wonders, Trade Routes, and a lot of peace.

• It's easier with a lot of civs,
  
• Only one opponent needs to "defend" with Culture output, vs. a Tourism civ
  
• Defend your own tourists by breaking trade routes with war
  
• Capture a city, remove all resorts(does not cost builder actions) before ceding it, or even switch tile ownership if you have a nearby city[/i] switching ownership was considered an exploit and removed
  
• [switch]Nuke Wonders that hold Great Works to instantly turn them into rubbish[/switch] Wonders are now immune to explosions
  
• To win, get all Works you can
  
• Trade/steal for theming bonuses
  
• Fill an archaeology museum or 2, no Great People hassle and they all give +3/+3
  
• Build Wonders
  
• Use your governors
  
• Make Parks, Resorts, and a lot of woods, Appeal matters
  
• Blanket your lands in Flight-activating improvements
  
• Create Entertainment buildings for that final bit of Tourism

Check out my Non-Cheese Deity Guide as well!