Take position with your back flat on the ground, hands behind your head and feet flat on the floor such that the apex created by your knee forms a right angle. If it feels more comfortable, you may instead cross the arms on the chest. This is the starting position.
Commence the movement by contracting your abdominals and breathing out as you lift your shoulders off the floor to the maximum height you can squeeze to. As you lift your body, your lower back should remain in contact with the floor. This contact remaining is what limits the height your shoulders achieve.
At the top of the movement, squeeze the abs hard before slowly lowering the body back to the starting position, inhaling as you do.