Set an Olympic bar on a squat rack so its high enough to hang under without touching the floor. Usually this means the bar would be at shoulder height.
Use wrist straps, with a wide grip, hang underneath the bar with legs extended and you body straight and stiff. Pull up until your chest almost touches the bar, then lower until your arms are completely extended.
Heels touching the floor is the easiest, try placing your feet up on a bench. Cross your legs to take them out of the movement.
Placing your heels on a swiss ball intensifies the exercise. To make it even harder, place a weight plate in your lap, or even a sandbag!