Bring up strength in the posterior chain. Using a box allows you to break up the eccentric and concentric phases of the lift and prevents you from using the stretch-shortening cycle. This will help you with force production out of the hole with your regular squat.
Set up in a wide stance to make sure you're targeting the hips and hamstrings. Then sit back on the box and make sure that your shin is vertical, not slanted with your knee forward. Don't let your back round forward when you sit down.
Use a 13-15 inch box. If you're a beginner, add a little height to the box by putting a plate on it.