Lie on the ground, facing up, holding a staff or band overhead, and take a hollow body position: tight core, lower back touching the ground, legs straight and off the ground.
Lift your legs up and make them go through between your arms, letting the object held go around and behind your legs.
Lower down your legs - the object held should now be behind your butt.
Lift your legs again to reverse the motion. You've completed one repetition after going back to the starting position.