You can use a chair, box, or a set of stairs for this exercise. You can also add a dumbbell for a little more of a challenge. Start with the right leg stepping up onto the platform and bring the left leg up, lifting your knee as high as you can. Step down with the left leg and bring the right leg down. Repeat with the other leg if the workout says to "alternate". If doing one leg at a time. simply continue using that leg for the allotted time.