Prepare using a rack with barbell situated at upper chest height, position bar on the back of the shoulders across your traps (Not your Neck) by coming up underneath and standing under the bar. Hold the bar with your hands slightly more then shoulder length apart to keep the bar balanced. Keep head forward, back straight and feet flat on the floor and positioned ahead with a slight outward angle as the below picture indicates. You're feet should be slightly more then shoulders length apart as well as your hands which hold the bar; equal distribution of weight throughout forefoot and heel is crucial. Dismount bar from rack and begin descent controlled and even. Execution: Descend until thighs are parallel to the floor at a 90 degree angle - toes should appear to line up with the knee before ascending back up explosively. The knees should not go past the toes when the thighs are parallel to the floor. Extend knees and hips until legs are straight. Return and repeat. This picture from the side indicates correct form - including foot position, knee position, parrallel positioning, and head/shoulder arrangements.