Lie on your back with your hands under your butt and your legs together. Tuck your tailbone and make sure your back is completely flat against the floor.
Lift both legs a few inches off the floor, making sure to keep your lower back flat and core braced.
Keep your right leg straight as you lift it up higher, forming a 90-degree angle with the rest of your body (or about as close as you can get).
Lower your right leg as you simultaneously lift your left, switching positions in a kicking motion. Keep your shoulders as relaxed as you can, holding all the tension in your core.