With a barbell in the front rack position elevate the rear foot, no higher than the knee, and bring your torso as upright as possible. As you begin to squat, pull the hips slightly back and don’t let the front knee run out in front of the toes. Go as far down as you can control the movement and drive yourself back up.
Elevating the rear leg makes the front leg work much harder. If you cannot control the knee from traveling forward or dropping inward, decrease either the weight or the height of the box.