Lie down on your back with arms at 90 degrees out from your shoulders, palms and arms pushed actively into the ground to stabilise your shoulders and spine.
Lift your legs off the ground, bending your knees at a 90 degree angle like you're sitting in a chair.
In a slow and controlled motion, lower your feet to one side. Pull your belly button to your spine. Your spine will lift slightly off the ground as you turn, but try to press it into the ground as long as possible.
Engage your abs to lift your legs back to the starting position.
Repeat, lowering to the other side. This completes one rep.