1 Sit down, place a step in front of your bench, and hold a pair of dumbbells. Place one dumbbell on each knee while setting the balls of both your feet on the step. Without touching the floor, lower both heels as far as you can. 

2 While pushing off the ball of your left foot, lift your left heel as high as you can. Then, as you lower your left heel to the starting position, raise your right heel.

3 Alternate between the above until you finish the set.