Put all the weight in your left leg, keep both legs straight, and lift your right leg directly behind you. Let your glutes do all the work. Keep your right 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 right leg so it's parallel with your left, but don't let it touch the ground. This completes one repetition.