Stand on bench or narrow platform, with feet side by side.
Step down with first foot to side of elevated surface onto floor. Stand back up on elevated surface by straightening upper leg and pushing body upward. Place foot to original position next to opposite foot. Step down and back up with opposite leg in same manner. Repeat by alternating between sides.
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 bench or step will make this movement easier.
For greater challenge, use taller bench or box. Barbells and Dumbbells can also provide additional resistance in same alternating pattern or one leg at a time.