This is not a guide on how to earn the achievements through gameplay.
There are several other guides which cover that topic.
If you're looking for one of those, I've found the Complete Achievement Guide by Vlad to be particularly well made.

Many people will consider the contents of my guide as cheating.
I am inclined to agree, if only because Steam allows players to compare their completed achievements with the global percentage completed.
However, the above having been stated, Witcher 3 is a single player game. Play and enjoy it in the ways you see fit.

• You will need to enable the debug console in order to use the method below.
  I did this by editing a specific configuration file, but there are also mods which enable it.
  
  
• As far as I am aware, there is no way to lose a Steam achievement once gained on an account.
  So do not use this method for achievements which you do not want to earn, or which you plan to earn later through standard gameplay or other means.
  
  
• As of posting, almost all of these are untested.
  This is because I either already have the achievement, or I'm planning to earn it through gameplay later.
  If you're interested in testing one or more, try them out and comment your results. I'll update the achievement and give you a thanks at the end of the guide.
  
  
• Some of the achievements are marked with an asterisk (*).
  This is because the Steam name, or unlock criteria, differs from that which I found while exploring the game files.
  I think I've matched them up correctly. But if an issue exists, it's likely contained in the marked achievements.
  
  
• The lists below contain some spoilers.
  In particular, some achievement descriptions are hidden until you unlock them in Steam, but are shown here.

Even though this is a very small change, it involves editing a game file.
So, standard warning, backup any game files you're changing before you change them.

I found an an in-depth guide here[commands.gg], and summarized it below.

1. While the game is closed, use Steam to open the Local Files for Witcher 3: Wild Hunt. (Properties -> Browse Local Files)
   
2. Navigate to: The Witcher 3/bin/config/base
   
3. Open the file general.ini with a text editor.
   
4. Find the line with DBGConsoleOn in it, and change it to:
   DBGConsoleOn=true
5. If the line mentioned in 4 does not exist, add DBGConsoleOn=true as the last last in the file.
   
6. Save and exit.

Once in the game, the tilde key (~) opens and closes the debug console. (At least for me.)

Achievements are listed as follows:

Name
Unlock Criteria / Description
Id
Untested (If Applicable)

The console command is:
testunlockachievement(<Id>)

To unlock an achievement, replace "<Id>" in the console command with the Id of the achievement and enter the whole thing into the debug console.

As example, if you wanted the A Friend in Need achievement, you would copy, paste, and enter the following into the debug console:

A Friend in Need
Find and free Dandelion.
EA_FreedDandelion
Untested

All In
Play three hero cards in one round of gwent and win the match.
EA_Allin
Untested

Armed and Dangerous
Find and equip all the elements of one set of witcher gear.
EA_FullyArmed
Untested

Ashes to Ashes
Complete the contract on Therazane.
EA_MonsterHuntDao
Untested

Assassin of Kings *
Take part in the assassination of King Radovid.
EA_CompleteWar
Untested

Bombardier
Collect the formulae for 6 different bomb types.
EA_Bombardier
Untested

Bookworm
Read 30 books, journals or other documents.
EA_Bookworm
Untested

Brawl Master
Complete all fistfighting quests in Velen, Skellige and Novigrad.
EA_BrawlMaster
Untested

Brawler
Defeat Olaf, the Skellige champion of unarmed combat.
EA_Brawler
Untested

Butcher of Blaviken
Kill at least 5 opponents in under 10 seconds.
EA_Swank
Untested

Can't Touch This! *
Kill 5 foes in a fight without taking damage (except for Toxicity) and without using the Quen Sign.
EA_Finesse
Untested

Card Collector
Acquire all gwent cards available in the base version of the game.
EA_GwintCollector
Untested

Dendrologist
Acquire all the Abilities in one tree.
EA_Dendrology
Untested

Environmentally Unfriendly
Kill 50 opponents using the environment (e.g. swamp gas, insects or objects).
EA_EnvironmentUnfriendly
Untested

Even Odds *
Kill 2 monsters you have a contract on without using Signs, potions, mutagens, oils or bombs.
EA_FundamentalsFirst
Untested

Family Counselor
Find the baron's wife and daughter.
EA_FindBaronsFamily
Untested

Fast and Furious
Win all the horse races in the game.
EA_NeedForSpeed
Untested

Fearless Vampire Slayer *
Complete the contract on Sarasti.
EA_MonsterHuntEkimma
Untested

Fiend or Foe? *
Complete the contract on Morvudd.
EA_MonsterHuntFiend
Untested

Fire in the Hole
Destroy 10 monster nests using bombs.
EA_FireInTheHole
Untested

Fist of the South Star
Defeat an opponent in a fistfight without taking any damage.
EA_FistOfTheSouthStar
Untested

Friends With Benefits
Complete the subplot involving Keira Metz.
EA_CompleteKeiraMetz
Untested

Full Crew
Bring all possible allies to Kaer Morhen for the battle against the Hunt.
EA_GetAllForKaerMorhenBattle
Untested

Geralt and Friends
Win a round of gwent using only neutral cards.
EA_GeraltandFriends
Untested

Geralt: The Professional
Complete all witcher contracts.
EA_CompleteWitcherContracts
Untested

Globetrotter *
Discover 100 fast travel points.
EA_Explorer
Untested

Gwent Master *
Defeat Tybalt and win the gwent tournament held at the Passiflora.
EA_GwintMaster
Untested

Humpty Dumpty *
Kill 10 opponents by knocking them off somewhere high with the Aard Sign.
EA_FusSthSth
Untested

Kaer Morhen Trained
Perform 10 effective counterattacks in a row without getting hit or parrying.
EA_TrainedInKaerMorhen
Untested

Kingmaker
Complete the subplot about choosing Skellige's ruler.
EA_CompleteSkelligeRaceForCrown
Untested

Let's Cook! *
Learn 12 potion formulae.
EA_BreakingBad
Untested

Lilac and Gooseberries
Find Yennefer of Vengerberg.
EA_FoundYennefer
Untested

Master Marksman
Kill 50 human and nonhuman opponents by striking them in the head with a crossbow bolt.
EA_TechnoProgress
Untested

Munchkin
Reach character development level 35.
EA_Immortal
Untested

Mutant *
Fill all mutagen slots.
EA_TrialOfGrasses
Untested

Necromancer
Help Yennefer extract information from Skjall's body.
EA_YenGetInfoAboutCiri
Untested

Overkill
Make an opponent suffer from bleeding, poisoning and burning simultaneously. Do this 10 times.
EA_Rage
Untested

Passed the Trial
Finish the game on any difficulty.
EA_FinishTheGameEasy
Untested

Pest Control
Destroy all monster nests in the Velen / Novigrad region, or in Skellige.
EA_PestControl
Untested

Power Overwhelming
Have all possible Place of Power bonuses active at the same time.
EA_PowerOverwhelming
Untested

Ran the Gauntlet
Finish the game on the "Blood and Broken Bones!" or "Death March!" difficulty levels.
EA_FinishTheGameNormal
Untested

Shrieker
Complete the contract on the shrieker.
EA_MonsterHuntFogling
Untested

Something More
Find Ciri.
EA_FindCiri
Untested

That Is the Evilest Thing
Ignite the gas produced by a Dragon's Dream bomb using a burning opponent. Do this 10 times.
EA_TheEvilestThing

The Doppler Effect
Resolve the doppler problem in Novigrad.
EA_MonsterHuntDoppler
Untested

The Enemy of My Enemy
Use the Axii Sign to force one opponent to kill another. Do this 20 times.
EA_EnemyOfMyFriend
Untested

The King is Dead
Defeat Eredin.
EA_DefeatEredin
Untested

Triple Threat
Kill 3 opponents in one fight using 3 different methods (sword, bombs, crossbow, signs, fists, environment).
EA_Cerberus
Untested

Walked the Path
Finish the game on the "Death March!" difficulty level.
EA_FinishTheGameHard
Untested

What Was That?
Attack, counter, cast a Sign and throw a bomb (in any order) in under 4 seconds.
EA_LearningTheRopes
Untested

Woodland Spirit
Complete the contract on the Woodland Spirit.
EA_MonsterHuntLamia
Untested

Xenonaut
Visit Tir ná Lia and convince Ge'els to betray Eredin.
EA_ConvinceGeelsToBetrayEredin
Untested

Can Quit Anytime I Want
Be under the influence of seven potions or decoctions at the same time.
EA_Thirst
Untested

Curator of Nightmares
Recreate all of Iris' nightmares in the Painted World.
EA_TheCompletePicture
Untested

I Wore Ofieri Before It Was Cool
Collect all available Ofieri armor and horse gear, and at least one Ofieri sword.
EA_LatestFashion
Untested

I'm Not Kissing That
Kill the prince cursed into a toad.
EA_ToadPrince
Untested

Killed It
Win a round of gwent with a total strength of at least 187.
EA_KilledIt
Untested

Let the Good Times Roll!
Participate in all the activities at the wedding.
EA_PartyAnimal
Untested

Moo-rderer
Kill 20 cows.
EA_WantedDeadOrBovine
Untested

Pacta Sunt Servanda
Finish the "Hearts of Stone" expansion.
EA_HeartsOfStone
Untested

Rad Steez, Bro!
Slide downhill uninterrupted for at least 10 seconds.
EA_Slide
Untested

Return to Sender
Kill 3 opponents with their own arrows.
EA_FeatherStrongerThanSword
Untested

Shopaholic
Buy all the items put on the block at the Borsodis' Auction House.
EA_Auctioneer
Untested

When It's Many Against One... *
Provoke all Iris von Everec's Nightmares into fighting you at the same time and defeat them.
EA_KillEtherals
Untested

Wild Rose Dethorned
Destroy all the fallen knights of the Order of the Flaming Rose's campsites.
EA_DivineWhip
Untested

A Knight to Remember
Obtain a flawless victory in all the competitions during the knights' tourney.
EA_ChampionOfBeauclair
Untested

David and Golyat
Kill Golyat with a crossbow bolt to his eye.
EA_Goliath
Untested

Dressed to Kill
Unlock the bonus for equipping all the witcher gear elements from one School.
EA_ReadyToRoll
Untested

Embodiment of the Five Virtues
Be given Aerondight by the Lady of the Lake.
EA_LikeAVirgin
Untested

Hasta la Vista™
Kill a frozen opponent with a crossbow bolt.
EA_HastaLaVista
Untested

I Have a Gwent Problem
Collect all the cards in the Skellige deck.
EA_GotToHaveThemAll
Untested

Kling of the Clink
Serve time in Toussaint.
EA_BeauclairMostWanted
Untested

Last Action Hero
Be decorated with the Order of Vitis Vinifera.
EA_HeroOfBeauclair
Untested

Playing House
Use all available options for developing Corvo Bianco.
EA_HomeSweetHome
Untested

The Grapes of Wrath Stomped
Help unite the warring vineyards and have wine named in your honor.
EA_BloodAndWine
Untested

The Witcher's Gone South
Travel to the Duchy of Toussaint.
EA_BeauclairWelcomeTo
Untested

Turned Every Stone
Find all grandmaster diagrams for each witcher school.
EA_TurnedEveryStone
Untested

Weapon "W"
Develop a mutation.
EA_SchoolOfTheMutant
Untested

Thanks to come as helpful folk test these out.

Please comment if you have a question, spot an issue, or discover something which should be updated / changed.

Cheers!