Get supine on the ground, hands under your butt.
Raise your legs off the floor and bring them above your hips. Tuck your chin to your chest.
Bring the legs down in a large ‘C’ shape (both feet making a circle). Your feet should come together just above the ground.
Raise your legs back up and repeat.
To make it more difficult, do the opposite. Start at the bottom, raise the legs in a ‘C’ shape and then lower together. Repeat.