You don't need a fancy CrossFit box to do these, you can use a staircase or a sturdy chair to make these elevated step-ups your glutes' best friend.
Stand in front of your chair, box, or steps. Lift your left foot, and make sure that it's at about a 90-degree angle.
Step your left foot up onto the flat platform, press down into the surface, and lift up through your right foot until your right knee is at a 90-degree angle to your hips.
Return your right foot to the ground and switch feet, repeating the process starting with your right foot on the platform.
Make sure to drive the lead knee up as you step up, squeezing your glutes and tightening your abs each rep.
Start with 3 sets of 10 on each side, and add on more reps as your endurance increases.