Lie flat on the ground with your legs extended straight up toward the ceiling. Place your hands underneath the back of your head. For added stability, you can place your arms out to the side, allowing your palms to grip the floor.
Pressing your lower back into the ground and keeping your navel pulled into your spine, slowly lower both legs down toward the floor as low as they can go without breaking your form (and without touching the ground), then slowly raise them back up. If this is too difficult, lower one leg at a time.