In order to master FPS, there are two very important aspects that must be learned and practiced. Aiming. Placement.
Using these two aspect in unison, you will surely succeed.
Placement can range anywhere from standing to the side of the hallway instead of standing in the middle of it, so that if someone comes down the hallway, you will have more time to react, to being in combat and knowing whether you should jump or duck depending on what weapon you have and your enemy has. You should also think about going behind enemy spawn points to abuse their focus on your allies.
Aiming, of course, would be knowing how to hit the enemy and how many shots it will take, but also, in aiming, we need to know when to knife and switch our weapons.

Placement:
Perhaps the most important aspect of gaming ...
Let us numb this down even further into a few types of strategic placement types. Camping, Sniper Camping, Counter Camping, Front line(In the open),

Camping: Personally, I don't camp - I'm quite a radical gamer(I acog scope my Intervention), but I'm not going to lie, playing like that will get you killed. Camping is basically waiting behind a door for someone to come in and blast them. Sounds easy, doesn't it? Well, I've killed a fair share of campers, I'd say, so why is this? COD: We have tools, like our UAV, our heartbeat sensors, and our personal knowledge of camping
spots; but even with this, campers are still able to back stab. When you camp, you should always have your gun facing the door(if there are multiple door, put your cursor in the center of each), you should always listen for footsteps and gun shots.
Now, when leaving the room for something, such as ammo, or a kill, watch your **** corners and your map. If you're chasing someone in a building, if he hears you, he will try to camp for you(see counter camping).
If you want to camp effectively, you will need to whatever building enemies will run into...camp in one building until the front line shifts, then leave and camp in another building. It is a sure way to get kills, but you have to have patience. Harness Cover and get low. Predict when someone will be throwing a grenade into your camping spot.

Sniper Camping:
Sniper Camping is essentially camping, except, with the use of a scope, you're camping long distance. Not that you can't sniper camp with a rifle, Sniper Campers are more prone to death from other snipers and an unguarded behind. If you have claymores are teammates, you're good.

Counter Camping:
Counter camping are the methods in which we eliminate campers.
If you can, while watching your back, distract a camper or hinder him, it would be a good time to run in, as long as he either can't see you or his gun is not pointed at you. At fast pace, it will be hard for the plan to go exactly like this; I've thrown some flash bangs at people and a box they hid behind conveniently protected their eyes. So why even run in at all if you're risking death? It is best to throw a grenade(after hindering if possible) instead of yourself.
Or, you can just run in and try to shoot him first which won't work if you don't immediately know where he is hiding. UAV, Heartbeat Sensors, Prediction, are all good tools to find out where an enemy is hiding, and if you're a hacking, you always know where they are(not a fan of video game hacking).

Front Line:
If you find yourself without cover, being shot at, I've learned that running works sometimes, but depending on the situation, you might want to fight back. The situations you'd want to run in would be when the enemy is far away and there is close cover, or whenever the enemy will certainly kill you and you cannot fight back. When you run, though, you'll never prove to yourself that you could have owned the **** out of that guy, even though the chances were way off. So, if you want to be a badass, you should fight back. In most games, getting headshots is a one hit kill, opposed to the enemy who has gotten 3 shots on you and needs to fire one more, in that one shot, you could win; I will not claim I've ever done this, though, and it is a mere fantasy, but I propose things that come close to that happen when you decide to fight. This is where aiming comes in, Sensitivity to be specific. You can abuse the enemy's medium sensitive controller by doing a quick prone/crouch and killing him while he aims down; You'd be surprised at how much time this will often buy.
Anywho, lets talk about multiple enemies in this no cover situation; You should probably prevent ending up in this, but, again, depending on many things, you can achieve victory. If you have no cover and the enemies run around a corner, it depends on your weapon. If you have a machine gun, you know what to do, if you have a rifle ...
Well, if you can try to manipulate the enemies around your cover by moving in (for example, I am in the middle of a hallway and the enemies come around the corner in front of me. If I move onto their side of the hallway on the opposite side of the wall, I can take some of them out of my view, they will have to clot into a line, and they can be shot accordingly; Thinks may go by to quickly for you to do this, though.), they will line up, and so, instead of shooting one enemy at a time, which certainly won't go well, you can shoot them all at a time(they also block each other's view.). Thinking in rational ways in combat to defeat opposition has been militarily proven to be effective, and most FPS are based on the military.

Aim

Aiming(Under Pressure)
I see this alot. Two guys walk around a corner, one guy has his aim on the other guy, and the other guy thinks he has his aim on the guy, both fire, guess who wins. I know, we will all fire when we think we're going to hit them, but you might kill an ally, and if you have bolt action, you're ****. If you see you're not hitting them, just remember, check your aim.
Knifing is an issue I have. I don't knife people whenever I walk around a corner; I usually shoot a semi auto rifle, backing up, trying to no scope them, but they can stab me simply because of bad reaction.
Switching weapons is useful, depending on your weapon. If you're low on ammo, or out of ammo, switching is usually the best thing to do, but it takes a couple of seconds to switch on COD depending on the weapon

Sensitivity:
I play on High Sensitivity, but everyone is different. The higher sensitivity you have, the easier it is to get to your target, but not precisely. I said before, sensitivity can be abused, but it isn't always smart to assume someone' sensitivity, though I'm sure 80% of people don't even change it.
It is worth fooling around a little, perhaps you will like a new sensitivity.
If you tried a higher sensitivity, I must tell you, you have to play for a couple of hours to get used to it. Your muscle memory makes you turn around a little too much, etc.

Coping With Lag:
If you have to aim in lag, I suggest finding a sensitivity that is smooth, and fast. It is a real pain in the ass ... The enemy has an unfair advantage over you.

QuickScoping, Drag Scoping, Hard Scoping[Snipers];
Ahem; QuickScoping is whenever you, for a split second, fully scope(I see some people scope half way; I have learned this does not work well long range - Best to fully scope), get your sights on the target, and kill them just like that. When this is mastered, Quick Scoping is a long range shotgun, almost(Not that it is bad a close range, though, if you miss close range, you're pretty much ****). QuickScoping Usually takes me less than a second to kill a target(After discovering him)

Drag Scoping is a little bit slower than quick scoping, though, there is more time to line up your shots and predict enemy movement. Drag Scoping is when you see an enemy in the field, scope somewhere not very close to him and drag it towards him, which works just fine. It will take me 2 Seconds or so to drag scope someone(May vary from sensitivity) . Some might argue that Drag Scoping is the same thing as Quick Scoping - Personally, I disagree, as there is enough time to die in between the two.

Hard Scoping: Hard Scoping. Hmm.
It is the manner of sniping that probably requires the least skill, though, you are always unaware of what is going on around your sniper scope - It is best to unscope on occasion to see what is around you, though, try not to lose kills.
Hard Scoping is when you keep your scope up, looking down a hall or waiting for someone to reappear. If you are a quick-scoper, it is certainly okay to hard scope if you need to. If I am waiting on enemies who know I am here, if my back is covered, I really don't have a problem with hard scoping their asses for a bit.
As I said, Hard Scoping is the easiest. You can kill a target faster than quick scoping while hard scoping(Once the enemy has been discovered)
Align the shot and fire, that is all.

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This guide is as detailed as i could get it.
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Con
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