Start by lying flat on your back. Gently press your spine into the ground and lift your legs off the ground, keeping your legs straight and lifting until they're above your hips (or as close as you can get) with your feet flexed. Keep your head and neck resting on the ground and your palms by your side. This is your starting position.
Inhale as you lower both legs straight down toward the ground, stopping at about halfway down. Make sure your back stays completely flat against the ground.
Exhale as you bring your legs back up to return to the starting position. If you want to make this move more challenging, use your abs to lift your hips an inch off the ground at the top.
This counts as one rep. Do as many reps as you can with proper form in one minute.