The question is "How do you land an airplane?" First of all, the chances of you being asked to land an airliner are zero, zip. It only happens in Hollywood. I would need to write a full sized novel to tell you how to land a commecial airliner. We will therefore limit this discussion to small, private aircraft.
So there you are staring out the window as the pilot finishes off that three day old turkey sandwich and keels over. What do you do?
Step 1: Communication
Step 2: Controlls
Look outside. Try to familiarize yourself with the relationship of the nose of the aircraft and the horizon. Look to your left and right at the relationship of the wingtips and the horizon. If we assume for now that you were in straight and level flight when the pilot ate his sandwich, the wings will be about the same distance from the horizon. This "picture" is something that will help in the general control of the aircraft. Now look over at the instruments in front of the pilot. These instruments are usually arranged in a standard configuration. The top left gauge should be the airspeed indicator. It will have numbers ranging from 0 up to a little more than the maximum speed of the aircraft in a clockwise direction. If you're in cruise in a small aircraft, its needle is probably pointing at a number around 120 to 150. This is your current airspeed in knots. The next gauge to the right of the airspeed indicator is the artificial horizon. Some people call it an attitude indicator. This gauge shows the relationship of the aircraft to the horizon. (Just like that "picture" you saw when you looked around outside.) The next gauge to the right of the attitude indicator is the altimeter. This gauge shows your altitude above sea level. If the needles are moving clockwise, you're climbing. If they move counter-clockwise, you're descending. The longest needle indicates feet in one hundred foot increments. The next longest shows feet in one thousand foot increments. If this needle is pointing between the 2 and 3, and the long needle is pointing at 5, you are at 2,500 ft above sea level. The next gauge to look at just below that attitude indicator is the compass. It may have a bunch of fancy needles in the center or sides of the compass card. Just realize that the compass is read at the top of the gauge.
The next things to look at are a series of knobs that stick out along the bottom of the panel to the right of the pilot's seat. In a basic airplane there will be three push-pull knobs. The largest one is usually black and closest to the pilot. It may be labeled "throttle". This is the one you'll be using. Pushing the knob in gives more power, pulling it out means less power. To the right of the Throttle is the mixture control. This knob is the same size or smaller than the throttle and is usually red in color. It should be labeled "mixture". Pulling that knob out kills the engine, so leave it alone. The next one over is labeled "Carb Heat" or "Carburetor Heat". It's a small knob usually black in color. You'll pull this out later on. More advanced aircraft have an additional knob located between the throttle and mixture controls. This knob controls the pitch (the bite the prop takes with each rotation) of the propeller. Leave it alone. You'll push it in later. A twin engine aircraft would have two of each of these knobs.
Step 3: Maneuvering
Climbing and descending can also affect the speed of the aircraft. One way to understand the effect of climbs and descents on airspeed is to think of a roller coaster. When a coaster goes downhill it picks up speed, uphill it loses speed, because of gravity. The same can be said for an aircraft. To compensate for this we'll use that big black knob labeled "throttle". Just as you would take your foot off the gas in your car going downhill, simply pulling on that knob a bit will reduce the power produced by the engine and allow you to go downhill with little or no increase in airspeed. The opposite is true of a climb.
Step 4: Landing
It's time to land so we must slow down. To do this you must understand that your wing generates lift (the force that keeps you in the air) in direct proportion to the speed of the air flowing over it and the angle (angle of attack) that it has with that airflow. If you decrease one (airspeed) you must increase the other (angle of attack) to generate the proper amount of lift. So pull the throttle back about half way, and gradually pull back on the control wheel without climbing. You will see the airspeed begin to slow. If take another look at that airspeed indicator, you will notice concentric green and white arcs. The white arc brings up a discussion of flaps which we won't get into. Look at the green arc. The bottom of it is the slowest airspeed that the aircraft will fly without the use of flaps which will be just fine for your landing. On a typical light aircraft this speed will be around 70 knots. To safely fly the approach, pilots add a fudge factor which is typically 30%. In this case you'll target around 90 knots for your approach. Once you reach 90 knots lower the nose a little bit. If you lower the nose too much the airspeed will be too high, not enough and the airspeed will be too low. This will begin a controlled descent, or "glidepath" to the ground. If your aircraft has that little black knob labeled "Carb heat" pull it all the way out. This prevents your carburetor from icing up (a bad thing). If your aircraft has retractable landing gear, look for a switch with a small tire on it and move it to the down position. If you can't find it, don't worry about it. You don't care if the airplane ever flies again when you're done. You just want to walk away. If you have one of those propeller controls (smaller black knob next to the throttle) push it all the way in. Roll the wings left or right to keep the aircraft lined up with the landing surface. You want your target spot on the runway to stay in a fixed spot in the windshield. If it seems to be going under the aircraft (you're getting high), pull the power back some more and make an adjustment with your nose (lower it) to maintain your speed. If your target spot seems to be going away from you or rising in the window (you're getting low), push the throttle in (increase power) and adjust the aircraft's nose (raise it) to maintain your airspeed. As you cross the threshold of the runway you should be close to the ground. At this point you'll want to pull the throttle all the way out and pull back just a little bit to keep the nose from dropping. Once the aircraft makes contact with the surface, it may bounce. Resist the urge to fight the bounce. Just hold the nose steady and it will eventually stay on the surface. If you're an ace, and your wheels are down and in one piece, let go of the controls and use your feet on the pedals to steer. If you push on the tops of both peddles you will apply the brakes. The right pedal top controls the right brake; the left pedal top controls the left brake. Once you come to a stop, look for that red knob labeled "mixture" and pull it all the way out. This will kill the engine. Turn the key, if there is one, to off. Change your pants and exit the aircraft. Congratulations! Any landing you can walk away from is a good one!