Start lying face up on the floor with your legs extended toward the ceiling and your arms extended down by your sides, palms pressed into the floor. Engage your abs to curl your head and shoulders up off the floor.
Bend your knees, then extend them long, hovering them a few inches above the floor. Keep your lower back pressed into the floor.
Bend your knees and draw them in toward your chest, then extend your legs toward the ceiling, lifting your hips an inch or two off the ground.
Lower your hips back to the floor with control. That's one rep.