The air squat is performed by keeping both feet flat on the floor, shoulder width apart or wider, with the knees and feet pointing in the same direction. Keeping the back straight and eyes forward, squat down, dropping the hips.
At first only perform squats so that the thighs do not go further than parallel to the ground. Once you are certain you are executing squats with good posture and technique, you should try to go down as far as you can, touching your thighs to your calves with your heels flat on the floor. Be sure your knees continue to go in the direction your feet are pointing, and do not lean forward. Likewise do not let your back round.
If you are attempting weighted back squats, where you hold weight on your shoulders behind your head, your thighs should go no further than parallel to the floor during the exercise. FitDaily recommends front-weighted squats over back squats. By holding the weight on your shoulders in front of your head, you put far less pressure on your spine.