Lie down on your back with your arms extended out like you're in a "T" position, palms and arms pushed actively into the ground to stabilize your shoulders and spine.
Straighten your legs, and flex your feet. In a slow and controlled motion, lift your feet and legs up, then lower your feet and legs to the left side. As much as possible, maintain a 90-degree angle at the hip throughout the movement. 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 right side. This completes one rep.