Stand tall with your feet spread shoulder-width apart. Lower your body as far as you can by pushing your hips back and bending your knees. Pause, then slowly push yourself back to the starting position.
Your feet should be just outside of your shoulders for a 'basic' squat, with feet turned slightly out (15-30 degrees). You can shorten or widen your stance depending on your training goals, as different stances will stimulate your glutes and leg muscles in other ways.
Your knees need to be centered over your feet. Draw an imaginary line from your heel to your toes - this is the line your knees should follow when squatting. Don't let your knees collapse inwards. Let your knees move forward naturally to keep the back straight.
DEPTH OF THE SQUAT
Make sure that you lower your body to 90 degrees, until your thighbone is parallel to the floor. If your squat stays too high, you won't engage your glutes or your upper hamstrings. Depth depends on hip flexibility, so if your hips are tight try standing with your feet a little wider apart. If your hips are flexible, try squatting deeper to engage all of the muscles in your legs and help build knee strength.
BACK AND CORE
Your back must remain straight - by this we mean that it should follow its natural curvature. Don't hunch forward as you lower yourself into your squat. Try thinking "chest up" and keeping your eyes focused straight ahead. Also don't allow your back to arch unnaturally, as this adds extra strain on your lower back and risks injury.