Set-up: Position a stable box or jumping platform next to you. The platform should be roughly knee height. Standing sideways to the platform, brace through the core, pull your shoulder blades back and establish a neutral spine position. Lift your foot which is on the far side of the box and cross it in front of your other leg onto the box.
Movement: Place your weight across the heel of the elevated foot, press through it, and raise your other leg up and onto the box. Drive through the raised leg and do not attempt to push off using the other side. Make sure that the knee of your elevated leg tracks in a line with your toes. Now lower the off leg to the other side of the box until it is touching the floor. Engaging the adductors of your raised leg, once more press through the heel and raise the off leg back above the box. Now return to the start position under control. Be sure to repeat the exercise with the opposite leg.
Repetitions: Perform all prescribed reps on one side, then on the other. This counts as one set.
Optional weighting: When you are ready (and there is no reason to push for high weights on this exercise), the movement may also be weighted with dumbbells, barbells, chains and weight vests. If you have access to them, the chains and weight vests are the least obtrusive method.
Assists: While starting the movement it may help to use a shorter box and build up the height over time.
Benefits: The crossover step-up is an excellent exercise for training the glutes in their roles of hip adduction and abduction as well as rotation. It is also a great frontal plane exercise for athletics.