Set your box to the same height as your box squat. Set your ground foot out in front of the box slightly so you have to sit back to reach the box. Reach your hips back and lean your torso forward until you are sitting on the box, pause on the box than stand up as forcefully as you can. Control yourself all the way to the box, DON'T 'plop' onto the box.
As you are sitting make sure your knee is slightly rolling out, NEVER let it cave in. Reach your hands out as you sit to help with balance. Make sure your foot stays flat on the floor and your knee stays behind your ankle.
To add resistance:
- hold a plate
- put a chain on your neck
- put a band under your foot and around your neck
- add a weight vest
- add a yoke bar
With athletes, I typically do a single leg variation once a week. It's important to train it but doesn't need to dominate your whole program.