Position yourself so that you are hanging from the top of a squat rack or chin up bar. Your arms should be almost fully extended.
Your feet should not be touching the ground so depending on how tall you are, you may have to bend your knees a bit. Bring your lower body up trying to bring your knees up to your elbows.
Pause at the top for a one-count, return to the starting position and repeat. Try not to swing as you raise your lower body. Use your muscles, not momentum from a swing.