Put all the 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. Then lift it back up behind you.