Put all your weight in your right leg, keep both legs straight, and lift your left leg directly behind you. Let your glutes do all the work. Keep your left foot flexed, and work on lifting your heel as high as you can without leaning your torso too far forward. Engage your abs to help you stay balanced.
Then, lower your left leg so it's parallel with your right, but don't let it touch the ground.
This counts as one rep. Complete two sets of 10 reps on each leg.