Set the rack height so the bar is at mid-sternum. Stand with your heels shoulder width apart, toes pointed out at 30 degrees. Carry the bar on top of the posterior deltoids ("low bar position"). Hold the bar with a narrow grip (elbows in). Keep the thumbs on top of the bar. The wrists should be in a neutral position. After lifting the bar out of the rack, secure your position by lifting the elbows and lifting the chest simultaneously. Tighten the shoulders and torso.
While squatting, look down at a spot on floor about six feet in front of you. Squat to full depth (below parallel). The bar should stay centered over the mid-foot. Keep your back arched during the movement.
As you go down, the knees should stay in a parallel line with the feet. Keep your muscles tight so you store elastic energy. That means don't go down too fast. Bounce out of the bottom of the squat by bouncing off the hamstrings and adductors, not the quads. As you go up, drive your butt straight up in air (up, not forward). You are using the hips to drive out from the bottom. The hips and shoulders should rise at same pace.
The angle of the back in relation to the floor is determined by keeping the weight of the bar centered over the mid-foot. Keep tension on the posterior chain as you squat.
Do not allow the knees to travel forward as you go up. Do not allow the knees to collapse inward at any time. Make all knee travel occur in first half of descent.
You can also get this information in a 3" x 5" squat quick reference card.