This guide assumes you know the basics.

Your first choice will be the location of your first base.
The data:

1 base cannot give you full globe cover
You spot and intercept UFO nearby your base easier, faster, better
Each country pay you according to how good you perform for that country and how rich it is

The outcome:
You need your first base somewhere nearby lots of rich countries.

My choice:
I prefer Greece, as it covers all Europe, Russia, Part of China and most Africa. I believe it is the largest sum at end of month which is important for starting months.

What you need to do just after your base location selection:

Click UFOpaedia and read the contents. You can skip this but i reccomend you do at least 1st time.
Click Funding to check how much each country pays you to get an idea
Optionaly click Graphs to see the graphs - you will need that later when building more bases. They are empty now but will fill with time elapsed.

Click options. You have only choose base location so far so it is a good moment to save. I recommend the following saves for your game:
Month starts, you may need this if things go awfully wrong or want to change research or manufacture plan
Before intercept, you will need this to change interception plan if UFO is very strong to save your planes
Before skyranger, to correct equipment and soldiers or change skyranger landing plan (like abandon crash site because city terror occurs so you need to get there first)
Combat starts (this save will be from within the combat interface) to make sure you have a fresh start for any reason.
Personally i only use save to exit the game but thats me you better use several save slots to save wrong options at least (you may miss skyranger set up and sent only 2 soldiers with their fists for example)

Ok, now choices begin and you start plan ahead.

Click Intercept to check it, from here you intercept UFO with your planes and land your squad with the skyranger. You dont need it now. You can sent a plane for a partol if you like to see how it is. No big deal.

Now click Bases.
You see your base from top and another list of options.
Build new base, you do not need that just yet, you can't afford.
Base information. Take a look if you like.
Soldiers. Check them. You need to know at least their strength to make sure you do not overload them. If you do, the loose action points - mean they act less in combat round. They may not even be able to fire a shot. Very bad. 20-25 means load just a rifle, 26-30 means can take some more like 2 grenades or a medikit or a heavy weapon (cannon or auto) but with 2-3 clips max and no grenades or other. 30-40 can take on more, close to 40 can take rocket launcher with 4 rockets total for example and a pistol.
More than strength you can check their accuracy or anything else.
How you define your squad is up to you. What you need to know is that combat is hell difficult. My tactic is locate and safely destroy. I spot an alien with a squad member, this member takes cover ASAP and i eliminate the alien with ANOTHER squad member that the alien CANNOT SEE. This can be done with a grenade or a sniped shot for start. Grenade is easier from my prespective.

Equip craft. If you are happy with current interceptor loadout and skyranger combat squad and equipment ok. Else you need to change things buy purchase and sell gear. So you come back here later.

Research. Enter and select whatever you like. MediKit will give you a chance to save a seriously wounded soldier in combat. Serious wounds cause soldiers to loose HP per turn so eventually they die unless the mission is completed before. The medikit will save the wounded soldier if used on him.
The scanner is much like the motion sensor in alien, detect motion. Very usefull as it helps spot aliens before they spot you.
Laser weapons will open they way to research laser weapons for your squad, interceptor weapons and base defences. You can skip however and go directly for alien plasma weaponry. There are pros and cons.

I choose scanner, then medi, then laser. I do research and manufacture laser because i consider the advantages important and give me more FASTER than plasma. That is:

They are researched much faster than plasma.
You can manufacture them right away without elerium needed. Elerium is not abandon nor you can manufacture. You only collect it from UFO.
No ammo clip needed.Perhaps the greatest advantage after faster research.
You can manufacture faster and cheaper and without elerium means you can then sell to gain extra cash if you are short of.

Laser and then plasma or plasma directly is one of the most hard debates on X-COM. I have a separate section on that.

My view is that with laser weapons you make your life easier than without. But you are free to stick with starting weapons until plasma if you like.

Manufacture is not needed yet. When you research scanner or medi you will come here to maufacture some.

Up to this point you need to decide how to arm your interceptors, if you need more soldiers.scientists and engineers.

Lets go on equipment first.

My choice is avalance missiles x2 for interceptors. Also i need rifles, grenades and flares for my squad. I do not use heavy weapons or other gear but that is me. I use 6 soldiers only. Then i hire scientists for as living space i have available.

That means i equip 1 interceptor with 1 avalance i have, i select 6 soldiers for the skyranger and equip the rifles and grenades.

Then i go sell all stuff i do not need. i purchase 3 avalance launchers and 9 avalance missiles, 4 soldiers so i have another 6 at the base when the skyranger is out, the rifles i need, and enough clips, grenades and flares.

When suff comes i equip interceptors. I also add some flares to the skyranger. Do not worry about the base, if you get attacked while skyranger is out, you equip soldiers left behind with equipment left behind before batle. If you want you can buy a tank for base defence as well, or two.

After finish equipment on my interceptors and skyranges, i go and sell all not needed stuff.

Build facilities.
You need:
General stores. If you do not build at least 1 you won't even be able to store the loot from UFOs you intercept and clear out. Build at least 1 and check your stores capacity from base info menu.
Large radar system. You need that to scan UFOs further. Once build you can dismantle the small and build another one large. Facility boxes must be always linked make sure you build the general stores below the existing one to ensure base facility links. Each radar has a chance to detect, so 2 are better than one.
Living quarters. You need that to buy more scientists, engineers and soldiers. Research needs are very importand and high at start.
Laboratory: You need that to reasearch with more scientists.
Optionally you can build missile defences. I build them for story/logical reasons. The last hope on earth, the very first base will be not protected by aerial landing? Does not make sense. You won't get attacked that early without this to be a rule. There is low chance that you may have base attack on month 1. It is superhuman after all. If missiles hit, alien attacking force will be much lesser. All is chance in this game.

You do not need other facilities at month 1. However you can build whatever you like. Make sure you save some funds to run the month. Pity to need clips and grenades and be out of cash.

Let's see you make all choices up to this point.

You have decide:
Interceptor weapons
Skyranger combat squad
Skyranger combat squad gear
Combat squad and gear out of skyranger - for base protection or reserves purposes
Sell gear you do not need, if so
Purschase gear you need, if so
Start at least one research project.
Build some mandatory facilities.
Suck personell you do not need, if so (not common for start)
Hire more personell you need, if so (usually scientists and perhaps soldiers)

If you are done with the above you can go to geoscape and click one of the time scale buttons to run time faster.

When time runs fast you are ALWAYS prompt to act/return to 5sec scale. So do not worry about loose an event or action. All the game goes that way. Examples:

Most likely the first event will be that things you purschase arrive at your base, and prompt to go to 5 sec time scale or ignore. Select 5 sec scale, then bases, then equip your new gear, then sell stuff you do not need if such. Sell/Suck is instant. When you complete all your actions, go back to geoscape and run time again.

Another case is when new hired scientists arrive, or engineers or soldiers. Same story. Select 5 sec, enter bases and allocate your scientists to a research project, or your engineers to manufacturing project or your soldiers to skyranger and so on. Then back to geoscape and run time fast.

If a research project completes game prompts you again. You see the research results, then prompt to allocate new research project and even prompt to allocate manufacturing if the completed research leads to maufacturing, like medikit and scanner.

I set a special chapter for this because you may get discouraged before you even begin.

Combat missions in UFO are in random places, generated. The skyranger usually lands away from the crashed alien ship or objective but do not count on that. The rule is that exiting the skyranger is where the pressure is ALREADY ON YOU.

The soldiers on top of skyranger crew list are those who are close to the skyranger exit door. This is a thing you cannot change. For a game where you control virtually everything even the smallest absense of choice looks terrible. Do not worry, all X-COM veterans live with it, so you do. Make sure the first two or at least one is expendable. If this pisses you off the easier solution is to have a tank to exit and ensure soldier deployment. Tanks always go first.

If you do not use a tank, there is only 1 way to exit the skyranger safely:

Left line troop LOOKS (right click to diagonally square) diagonally RIGHT
Right line troop looks dianogally left
Left troop moves diagonally right (exactly where he was looking) 1 square
Right troop moves diagonally left (exactly where he was looking) 1 square
Left troop (now positioned right but i keep call him left) looks diagonally right again
Right troop looks diagonally left again
Left troop looks diagonally right one more time (imagine this works like a cars side mirror)
Right troop looks diagonally left one more time
Left troop moves NOT diagonally to the square left of where he looks. He must now be out of the skyranger door, right side, diagonally next to the back right skyranger wheel. Look full right.
Right troop moves the same way from his side. Not diagonally to the square right of where he looks. Look full left.

The above procedure will grant you the top chance possible to scan the area around skyranger for enemies without aliens suprise you. I am happy to learn a better one but i believe there is none.

As you see, all goes in couples of steps. Left right, left right and so on. That is becauee the two soldiers actually cover eachother. If you sport you are nearby the end of the level (tiles end in a line where there should be more) either left or right side you can prioritize differently. If for example you see end at left side you may change initiative to right first for even higher chance to detect enemy first since chances are aliens won't be at the edge. Just chances though.

If it is night you need to use flares, at least 4. Throwing them forward, left, right and back seems easy but it is only easy for the front flare. It is easier if you throw forward-left, forward-right, back-left and back-right instead.

If during the above you see an enemy then if he sees you you need to kill him, if not i advise to continue your exitting scan, because there might be more.

Keep in mind that looking around skyranger does not guarantee there are no aliens close. Soldiers cannot see pass solid objects. An alien can be behind a tall wall, into a house or perfectly behind a single tree (very unlucky that one because t oblock field of view it must be a straight line - the alien, the tree and the soldier square when you looked.)

One final thing. Many believe you need to deploy all your troops at first turn. That is not true. 4 soldiers can secure the immidiate area around skyranger.

Hint from http://gtm.steamproxy.vip/id/nurkka :
Aliens have full TU at turn 1. If you be idle at turn 1 and exit the skyranger at turn 2 aliens will have the TU they preserve from turn 1 movement.

Combat is the most ipmortant aspect on UFO
Late or soon chances are you intecept a very small or small alien craft, crash it and go clean up and salvage with your skyranger.

Research and manufacture as well as bases and choices will work with many different plans
In combat however, you need to be efective on superhuman.

The golden rules:

Your soldiers are EXPENDABLE. Accept the fact that you will loose few or some. Not at every mission and not always when you expect it, but YOU WILL and, most importantly, YOU NEED TO PLAN YOUR SACRIFICES.

Example:
First mission. You clean up countryside perfectly. Only the small crashed alien ship is left. But damn aliens won't get out. Note here that there are ways to lure them, like firing at the closed door. You are free to explore. Anyway, door won't open unless an ufortunate soldier enters. Opposite to UFO 2 terror from the deep, on UFO 1 you cannot right click a door open. You have to move inside. And if you save the soldier then what? Step outside? To many actions for reaction fire. So consider that soldier dead before entering will help your morale. HE IS DEAD. He will sacrifice himself for the greater good and purpose and save humanity.

Aliens CANNOT SHOOT OR MIND CONTROL OR ANYTHING UNLESS THEY SEE AT LEAST ONE SQUAD MEMBER. Forget mind control for now, it will come later on. So ... THEY CANNOT SHOOT A SOLDIER UNLESS THEY SEE HIM. It is also good to know that they are not smart either, they will not auto-shot that wooden wall you are behind to destroy it and kill you for example. Something that you may do for example with heavy firepower.

Example:
You see an alien. You hide. If you throw a grenade with another also invisible soldier or fire from very far away, the alien CANNOT REACT AND FIRE BACK AT YOU because simply he does not see anyone.

ALIENS HAVE REACTION TIME MOST TIMES. Reaction is when you save action points for the enemy turn. You have it, aliens have it. Aliens also uses it most of time. You also will according to your playstyle. But at start you are weak, no good aim, no good reaction stat, no armor protection, no good weapons... too risky. Aliens however will do. When an alien see you it takes chance to react WITH EVERY FURTHER ACTION POINT YOU SPENT. THE MORE ACTION POINTS YOU SPENT, THE HIGHER THE CHANCE FOR ALIEN TO SHOOT BACK. This means that you may try to fire, but if you do not kill be ready to accept fire back. And probably die. However you may perform a fast action to minimize chances. For example, crouch behind that stone fence will perfectly do.

Much like how real command do, you need to move and secure the area part by part. Secure means there is no alien at a sub area NOR IT WILLLGO THERE UNSPOTTED.

So at all times you have secured and uknown areas into the combat zone. Starting up, the entire zone is uknown. That means aliens can be AND WILL BE ANYWHERE. NEVER ASSUME THAT AN ALIEN WON'T BE 'THERE', unless you SECURE the area.

You already read the tactic about exiting skyranger as safe as (super)humaly possible so you start getting the idea.

How you kill aliens is another paragraph. Here i explain how you move and secure. So we assume you exit the skyranger safely.

At this point you have some good view of the area. Exiting the skyranger mybe fixed, but now nothing is fixed, everything is random. Environment is random, aliens are in random location and you need to secure a sub area. Remember, secure is 100% no alien existing or go into without you spot it (within its alien turn or, more safely, at your turn easily and safely).

You need to study the level very carefully. THE FIRST 10 TURNS OF EACH MISSION ARE OF CRITICAL IMPORTANCE.

If you are close to a level end, plan how you secure that. Securing a corner for example allows you to control your flanks and check ahead. If you secure a corner your chances to secure the entire area step by step are as top as possible.

Aliens move at their turn. They move like your soldiers, somewhat less because they reserve some shot for reaction. You can do that but i do not recommend for start as chances are against you. You need that later on. You have to make sure that while they move they will not detect your soldiers, but while you move you detect them and as safe as possible. This is what is all about. Who surprises who.

I do reserve some action points but not for reaction fire. I reserve them for taking cover. 4 soldiers are enough to secure the skyranger immidiate area. Move them in such a way so they explore and look as much as possible. Then take them into cover. That is a fair turn. Example:

You have a house very close up front the skyranger door. This is a nasty situation. Hide the two initial scout troops behind the back wheels. The are safe from behind (where the house is) because there is the skyranger ramp. Have a soldier run straight into the house to secure it. No much choice here, if an alien is into the house and sees your line of troops into the skyranger he will shoot at them. So send one to check the house, try to take cover for this poor one when close. Alternatively, you can also use a smoke grenade to block sight from that house and safe your troops.

The above example is the most nasty situation with little control. Because the house blocks your field of view but if an alien just step out can shoot at your entire squad.

All other situations have better chance to get handled. Move 4-6 troops at the first 3-4 turns in such a way as you see as many area as possible and use as effective cover as possible. REMEMBER TO RECHECK BLIND AREAS AFTER ALIEN TURN ENDS.

When you spot an alien two things are important. Where the alien looks at and where are the OTHER aliens. A common mistake is to focus an entire squad to kill an alien. Remember, your soldiers must be at cover when their turn ands, as better cover as superhumanly possible. You DO NOT NEED TO KILL THE ALIEN IF THE RISK IS TOO MUCH EVEN IF THE ALIEN SEES YOU. If you can hide your troops from alien side and the alien cannot reach a position to see and fire a soldier, let it play. Most likely, IT WILL MAKE IT'S POSITION WORST AND YOUR CHANCES HIGHER because moving towards your squad will give you another fresh turn to kill it.

The safest way to kill an alien, especially at the earlier part of the game where all chances are against you, is only one.

You need to spot the alien with one soldier, then hide that soldier. The alien may look the other way. If not, it sees your scouting soldier for a moment but lost him before get a chance to fire. So now the alien cannot see any of your troops. But you had spot it, and this is what is all about.

The safest way to kill now is a grenade. Study the area and plan where you throw a grenade from. Grenades go to an arc path. So you cannot throw a grenade if the arc is blocked. You cannot throw a grenade far out of a house if you are into a house for example. But you can easily throw a grenade inside a house from nearby a window for example, or on top of your cover like a wall or a fence. Using grenades a while you get experienced with. The good with grenades is that the kill is certain for starting sectoids and lot other early aliens(check grenade and weapon damages), if a second unfortunate alien is nearby joins the first one to grave, one soldier may prime them and ANOTHER may throw them (! this requires some caution and playing experience!) and finally soldiers tend to be better at throwing than shooting as stat.

Another safe option is sniped them out. It is not as certain as grenade however for early missions. You have to be sure that the sniper is out of the field of view of the alien. Of cource there must be line of sight, which means that the sniper must either be very far away (a bit further than the max view distance). To know how that is imagine your scout moves and the level is pretty clear of obstacles. As move forward you spot an alien. If you step back the alien cannot see you because you just got out of max view. Another troop from behind can also safely shoot. If there is no such distance available you can use a smoke grenade. In any other case the alien may and WILL shot at you even if he initialy looks the other way of your soldiers. Snipe them out comes a better choice with laser or better weaponry or with troops having 60+ shooting, prefferably 70-80 or better.

So remember your combat advantages:

Alens cannot shoot at you unless they see you, they need both line of sight and max view distance. You only need line of sight, distance does not matter.
Aliens do not take cover nor they will fire at your cover even if you just spot eachother, then you slip into cover. Most likely they move towards your last seen position. Else they move their default movement plan. But you can throw a grenade at them nevertheless.
Sometimes aliens get some real nasty positions, like moving around roofs of buildings, which gives them great view. Grenades can still be thrown on building roofs.

Usually the worst mid-game surprise.

You cannot be prepared exactly because when MC weapons are deployed against you that is where you start learning and researching this.

It's been 20 years since i completed the game so i do not remember research items and names or exact procedures.

For sure, all soldiers do have hidden PSI stats, 2 of them, one for defence and one for offence. PSI defence is the most important as MC gets very difficult. The problem is that you do not know that stat until you build the PSI Lab, a base facility that does exactly that, check your troops and i think train them as well.

However from the time you first encounter MC until you deploy a MC high resistance team on the field (and later a MC attack team on your own - here is where real fun begins!!!) to negate MC attacks, you will have quite a large timeframe with perhaps dozens of combat encounters where you have to deal with morale loss and mind control.

What to do from start until you encounter MC:

Do not limit to a single combat squad. While you build new bases (the faster the better) make sure you actively use and strengthen 2-3 or even more combat squads. After all you will have increased number of encounters anyway. That way you will be able to pick the more PSI resistant of all squads and form up a PSI champion complete squad when PSI lab comes available.

What to do when your main squad deals with MC first time:

Here is another top rule. THERE IS NO TOP SQUAD UNTIL PSI LAB BECOMES AVAILABLE. Your commander with the 57 kills may be YOUR WEAKEST TROOP.

Nevermind, when your top squad comes against MC enemies your have little choices:

Safe Choice, if available: Retreat and send a secondary squad to do the job.
If you want to save your squad 100% with no risks: Abord the mission even if you cannot take on with a secondary skyranger.

If you still want to fight, and for every fight you go before PSI Lab:
Drop any armament that can cause mass damage like rocket launcher or high explosives or alien grenades until your troops are spreaded.
Spread your troops as much as possible.
Hide your troops. To MC, any alien must see at least 1 of your troops. Then if MCed, more aliens may MC any troop the mind controled troop see as well. So it works as a chain. But if none of your troops is visible to the enemy, no MC may occur.
If a MCed troop returns to your control, disarm him and drive him back to the transporter.
If you can avoid a MCed troop do it but usually it is very difficult without put your squad into further visibility risk. Hard to say but if aliens spot you and MC one member, safest thing to do is kill him. Sad but true.

In general, fighting aliens that use MC on your squad is hell. Funny thing that how the tactic search, locate and safely destroy you used at first missions comes back again, this time as mandatory.

Unless you save-load all the time (i never do that as it breaks immersion) fighting against MC is... superhuman. Comes to hell when you have squad you want to keep safe and complete the mission at the same time.