This is a difficult exercise that is used to work the lower body without weight. This exercise takes great strength, flexibility, and balance.
Start with a shoulder width stance and your hands in front of your chest for balance. Hold one leg up off the floor while maintaining your balance. Begin the movement by squatting down through your knee; also, push your hips back so your knee does not go in front of your toes. Squat down until your hip breaks the plane of your knee, or you can go all the way down until your upper thigh touches your lower leg; this will be more difficult. Next, drive through your heel and raise your body back to the starting position. Repeat this motion until you have completed your set and then repeat with your opposite leg.
Look straight ahead when you do this to help maintain your balance.
Also, you can hold onto a stationary object with one arm until you get strong enough to do this without holding onto something.
A more advanced movement would be to hold a weight in your arms while you are performing the pistol squat.