Start on all fours, with your hands directly under your shoulders and your knees directly under your hips.
Keep your right knee at a 90-degree angle as you slowly raise your leg behind you until your thigh is almost parallel to the floor. Make sure your back remains in a flat position throughout the entire movement; the movement should be coming from your hip flexor and glute; it should not feel like you're kicking the sky with your foot.
With control, lower your leg down, mainting a 90-degree angle at the knee.