Set yourself up on a box under a bar. The box should be high enough to allow you to get into a partial squat and lock your arms out while hanging from a bar. Once in position set your shoulders by pulling shoulder blades down and back. Jump hard and simultaneously pull the bar towards your chest. The goal is to get your chest to touch the bar and descend slowly. Make sure your shoulders stay down and do not collapse inwards.
If you find yourself dropping uncontrollably spend a bit more time on easier pulling movements like the Ring Row.