Stand between two benches, one to each side.
Lift leg and place foot on bench to side and forward of straight knee. Stand on bench by straightening leg and pushing body upward. Step down returning feet to original position. Repeat with opposite leg, alternating between legs.
Keep torso upright during exercise. Stepping knee should point same direction as foot. A modest degree of knee rotation occurs during this movement. See Controversial Exercises and Rotary Force in Squat Analysis.
Using shorter benches or steps will make this movement easier.
For greater challenge, use taller benches or boxes. Barbells and Dumbbells can also provide additional resistance.