At the flight school people always ask me:
"Can I land a plane? I have X years of flight simulator experience."
Short answer is: history shows you will probably die. Probably not because it is difficult but because you don't know what you don't know. Flight simulators distort important aspects of landing airplanes: your awesome 200 degrees/3D vision, the muscle mechanics of flying and the notion of distance. In fact, flight simulators are harder than the real thing. Yet, many in the flight school I teach landed without the need for intervention in their first flight. I landed an airplane for the first time when I was 11 years old. With a bit of luck you can do this by yourself. So, in case of an emergency, this is what you could do...
(TL;DR at the end)
There are many types of airplanes, but when it comes to landing there is pretty much two classes: heavy and light. This is more related to lift/weight ratio (and wing type) than actual the size of the airplane. I will show you how you can land light airplanes in the easiest way possible, as long as first you open any flight simulator right now (xplane for iphone is ok) and understand the basic controls (pitch, roll, yaw) and the relationship between speed and angle of attack. Playing with it for 10 minutes (at low speeds) should suffice. The basic mechanics are good enough. Keep in mind that brains are incredible machines and can learn things automagically, but you need to stay calm. For instance, my wife learned to keep an airplane flying without one single instruction.
The pilot died, now what?
1. Stay calm, take control
Flying airplanes is easy, stay calm. Your only goal now is to take control. The first thing you will do is put your hands in the yoke (or stick). Do not make sudden moves. Airplanes are not like cars, inputs must be very subtle and smooth. If the airplane is not leveled you will instinctively and smoothly move the yoke to make the airplane level against the horizon. At this point the plane will be likely going up or down a bit (maybe you are not even aware of that), don't worry about this now. Check if the pilot's body is blocking or pushing anything, check if the pedals are clear. (don't touch the pedals). The throttle is the black (or gray) lever on the middle of the panel or between the seats. Now, push the throttle forward until the end to feel where the end is and than pull back 30% of that. You should be using 70% of the engine for now. That will prevent you from crashing for a while. If you are flying a propeller airplane check the RPM (just like a car, should be right in front of you). The RPM should be around 2.300. The speedometer and tachometer should have colors, do never let it get close to the yellow or red areas. (yellow = this is wrong, red = you are doomed)
*Read about stall and spin recovery at the end
2. Inform the situation
Put the headset on. There should be a BLACK push button at the right side or your yoke. Your radio should be set with an approach or center. Push the button (keep pushed) and speak slowing (but briefly) what happened, start by saying "PAN PAN" or MAYDAY (depends on the country). Do not lose more than 15-20 seconds doing this, we still need to find an acceptable airspeed for this airplane! Also, you will need to find a place to land, if you know where you are, great! Also, the guys on the radio will say which direction you should go. If no one answers to you just keep flying straight. (do not make turns yet) Ask them if there is any pilot that knows that specific aircraft around (should help you finding the speed).
3. Flying straight and airspeed
Now the speed. I don't know which airplane you are flying (you also probably don't) so you will need to test a reasonable approach/flight speed. We do this by using the airplane's attitude/angle of attack. Airplanes usually fly at a certain angle of attack, the slower you fly the greater the angle of attack (and engine power) needed to continue a leveled flight (because it increases the lift and drag). What we want is to find a speed where the airplane flies with very small amount of angle of attack. To do that you will accelerate the airplane to about 70-90% of the throttle. Now, looking at the altimeter, stay at that altitude and start pulling the throttle back (slowly) and watch closely the horizon (or artificial horizon) as soon as you need to push the yoke back to keep flying at the same altitude, check the speedometer: that's 90% of the speed you should be flying now. Accelerate a bit to get to the correct speed. Remember this: everything is subtle, do not push back the yoke to the point where you the nose is going up more than 6-8 degrees. If there is a artificial horizon your angle should be just enough to make a thin blue line between the piece presenting the airplane and the yellow/brown background representing the ground.
4. Making turns
Making correct turns are hard. It takes 20+ hours of instruction to teach students how to turn correctly and they still make turns that suck. You are not going to learn this. That said, this is how your turns will work: first, where do you want a go? If this is a 180 turn, look 90 do the direction you want to turn, find a reference (trees, etc - use the tip of the wing) and start the turn by very slowly turning the yoke to the direction you want. This is so delicate that someone looking at your hands would barely notice you are actually moving the yoke. As soon as your reference is on the other site (same position relative to the wing), you finished the turn 180 turn. The maximum angle of turning you will use for this entire flight is no more than 10 degrees. (tilt your head to the right/left a little, that's more than enough). You might get a bit dizzy, because you are making the turns wrong, ignore it.
Now, the guys on the radio guided you to the runway or you found the runway yourself. This is the part where flight simulators are useless. You should be higher than you probably think. Most people have a wrong perception of the height the airplanes approach because the size and direction of airliners passengers windows, that makes first time pilots come too low, specially flight simulator players.
You should be at least at 1000 feet above the ground. If you see the number 29.92 set in the altimeter, ignore the altimeter completely. Ask on the radio for this altimeter setting (change the setting by turning the knob on the altimeter). If you don't have a radio or GPS, try this: you should be high to the point where you can see cars but cannot possibly identify the specific color or model. This is around the 45th floor of a building. Remember: the altimeter is showing your altitude relative to a sea level configured by that number, not the distance to the ground.
Your approach will consist of getting the airplane at this height and 1 mile (or less) apart from the airfield, aligned with the runway.
Anyone who plays with a flight simulator should get to this point without any instructions, but now things will get stupidly fast. Adrenaline and not knowing what you are doing are the main reasons for this. To land you will have to forget everything you know about xplane of microsoft's flight simulator.
Using small movements you will keep the runway between your legs. Be patient, do only small corrections, if you over correct you will start zigzagging. Airplanes are like kayaks, they are always skidding and inertia make things take a bit longer, you need to wait for you input to make a diference. (this impression is actually caused by our notion of space)
We should find a distance between you and the runway that you could turn the engine off at your current altitude and still reach the runway. We can't do that now but, good thing about light airplanes is that they loose speed very fast. That said, you will stay at the current speed (or the speed someone tells you on the radio). If you can locate the control to lower the flaps to its next position, do it now. This will feel like the airplane is braking and it could gain altitude, keep you current speed, the flaps won't break.
Time to dive: you kept the runway between your legs, you are 1000 ft above the ground and the runway is 1 mile in front of you. You will point the nose of this airplane to the very beginning of the runway. The speed will start to grow, you will reduce throttle to keep your current speed, you will not overshoot the runway, don't worry about that. Keep your eye on the speed. Some people will feel the pressure changing in their ears, this is normal. You pointed the nose of the airplane to the beginning of the runway, but you won't be able to land there. You should cross the beginning of the runway at the height of a 4-5 store building and descending. When you reach the height of a common pole, cut the throttle completely, you will start to pull and reduce the descending speed. If you pull it sooner it will get ugly. The airplane should be high as a very tall person now. Do not let it land, smoothly keep pulling it more and more to try to keep this height. After a few seconds you will hear a buzz. That's the plane starting to stall, because you followed my instructions now you should be around 1.5 meters from the ground and the plane will land by itself. If you ever hear that buzzing sound and the distance between you and the ground is greater than a height you can fall on your feet, push the throttle to the end, do not pull the yoke until the buzzing stops get altitude and try everything again. If the airplane hits the ground, immediately cut the throttle. Some landings can be so hard that can hurt a bit.
After the plane is on the ground, it won't go straight, it will turn to the left of right immediately after you touch the runway. The pedals, which you didn't use until now, are also used to brake and control the airplane on the ground. Do not put you whole feet in them. Instead, you will put the tip of your toe in the lower part of it. (like kicking) Push the left pedal, the airplane goes to the left (vice versa). If you push the upper part of the rudder you will brake one of the main wheels. Unlike cars, every main wheel has its own braking pedal. Do not brake the airplane now, wait for it to get slower. When the airplane is slow, move the tip of your toe to the upper part of the pedal and push left and right, slowing and simultaneously. Controlling an airplane on the ground sometimes feels like driving backwards a shopping cart at 60 miles/hour.
You made it! Now just push buttons around and you will end up turning the engines off (red ones first).
Playing 10 minutes with a flying simulator will make you more comfortable maintaining the airplane in the air but won't help you on landing. The biggest mistake most first timers do is to coming too slow and too low for landing. Make sure you are high and glide to the ground without the need to use the throttle (but use it if you need it). Do not fear the ground and start to flare only when you are 10 meters high (same height as a pole), when you reach the height of a tall person, keep pulling until the airplane stalls. You have the option to give up before touching the ground, never try to take off after touching the ground. Do never, ever push the yoke when close to the ground, that's a certain crash. If you flew by 2/3 of the runway, apply full throttle and try again. If you are on a newer/larger airplane, you will need an pilot on the radio to help you, good news is that it could be possible to program the airplane to land by itself (most newer airplane can emulate ILS CAT III systems).
Remember: you have time and you can keep trying as long as you can keep the airplane flying.
* Recovering from a stall / spin
Stalling and spinning is 99% guaranteed to not happen to you, but if it does: If you somehow start hearing a buzzing sound, push the throttle forward, DO NOT PULL THE YOKE, wait for the buzzing to stop. Now, if the nose is down, cut the throttle (pulling it back) and level the airplane SMOOTHLY, and apply the throttle back. If right after stalling you start to spin, you will have little time to react, do this: full throttle, push the yoke to the front and pedal + yoke to the opposite side of the spin. This will convert your spin into a regular dive, after it stops spinning, cut the throttle and smoothly pull it back. If the spin get flat... not good. Remember: it is ok to stall when you are below 2 meters above the runway.
Here is a video of a landing, this is one of the hardest airplanes you can fly. You could easily double the speed and altitude and land safely. 2/3 of that altitude would cause a crash: