Use a box that allows yours thighs to be slightly above parallel when you squat down. The height of the box will be determined by your height.
Set up like you would for a regular front squat. Step back until you're standing close to the box. Drive your knees forward, chest tall, pushing up into the barbell, elbows up and sit down on the box until your butt touches, then stand back up. Repeat.
*Do not sit all your weight down and "rock back" - this video is more of a "touch-and-go" box squat
