On the box be as close to the bar as you can to make sure your form of the step down is correct and your upper body can be in a good upright form during the squat. Put your weight through the bar with both hands to 'lighten' your body and unload the knee as much as needed. Step down and back into a one leg squat, aiming to let the knee travel forward normally. To return, 'push up' with your hands to assist the leg and keep the load off the knee as needed.
Aim to to do the movement without pain. Put as much weight through the bar as needed to achieve pain free and normal ROM of the knee and hip. Progress to the regression or step down once normal weigth-bearing is achieved