Lie on your back, and reach your arms rigidly to your side, off the floor. Lift your legs off the floor, and point them, so they are at about a 45-degree angle. Lift your head, so your shoulders are off the floor as well.
Lift your upper torso off the floor, and bend your knees. You can lean back to make this move harder or come up more to make it easier. Slowly lower your upper body back down toward the floor as you straighten out the legs. Stop when your back is on the floor, but not your head, shoulders, or legs.