Start lying face up on the floor, with your legs curled in toward your chest. Hold your hands together in front of your chest and engage your core to lift your shoulders up just off the floor, forming a tight ball shape with your body.
Pressing your lower back into the floor, extend your arms and legs out into an "X" shape, keeping your feet hovering off the floor. (If your back starts to arch, don't lower your legs quite as far.)
Draw your arms and knees back into a ball. That's one rep.