Your hips should function just as they do in an Air Squat. With One leg extended in front pull your hips back while maintaining a neutral spine. You may reach in front to assist in your balance. Knees may track over your feet, but not past your toes and heels must stay on the ground.
As you lower yourself on to the box try to keep most of your weight on your feet.
Begin standing by keeping your torso forward and driving your weight into the heel of your foot.
If you find you are on the box and unable to stand on a single leg then stand with two feet, do not heave your arms forward for momentum.