Start with your feet in a bilateral stance parallel to the step you'll be lunging onto. Keeping the outside foot grounded and parallel to the step, lift your foot closest to the step and step up onto the step with the foot landing in a 90-degrees to your grounded foot. Imagine the surface you're stepping onto is somewhat unstable so that your foot firmly plants on the heightened surface. Maintaining your balance, step up onto the step pull your other leg up off the ground so it lands bilaterally with your step up foot. Slowly reverse the process with the foot that stepped up onto the step as the grounded foot and the other foot that stepped up last as the driver. Slowly lower the foot to a parallel position (starting position) to the step. Again, imagine the surface you are stepping down onto is unstable/slippery. Do not "dump" the foot on the ground. Make it a controlled landing. When the foot has stabilized onto the ground, bring the other foot down off the step parallel to the ground foot.