Barbell Squat

Preparation
Put the barbell behind your back. With a movement from your pelvis (not your waist), push your butt back, bring your shoulder blades together and pull your shoulders back. Lift your chin so that your whole back makes a line. Keep that line tight and try to stay as straight as possible throughout the exercise. Take a little bit of air into the stomach (not the lungs) and tighten your abdominal wall (as if preparing to take a punch) and hold it this way. Do not loosen your back, the abs or air while doing the exercise. You can only breathe when you are back up.

Movement down
The butt goes back and down, knees do not go forward. Keep the weight on your heels. Try to stand as straight as possible; do not bend from the waist. Your torso will slightly bend forward to keep your back straight. Only when you are almost at the lowest point, can you let your knees go slightly forward, but make sure you make that shift as small as possible. Go down till you feel the pressure in your lower back rising, which is usually when your butt is a little above the level of your knees. Do not go deeper than knee level.

Movement up
The chin and shoulders start going up. Push up and forward with your pelvis. Keep the weight on your heels.