While standing on the step with one leg, keep your support leg straight and your abdominals engaged. Then allow your leg that is hanging off the step to slowly fall towards the ground. Do this by allowing your pelvis to slowly drop down.
It is essential to keep your support leg on the step as straight as possible. Many people want to bend the knee to lower down, but lower down by letting the pelvis drop slowly. Your foot should not lower enough to touch the ground -- be sure to control the movement with a slow, steady drop.
When your pelvis drops down as far as possible, hold this position for a second or two, and be sure to keep your abdominals tight. Then proceed to the final step of the exercise.