Start in a dead hang position. Make sure your body is rigid. Initiate the movement by pulling your shoulder blades back and down towards your hips. Make sure your abs are on keeping your ribs from sticking out squeezing your butt. Pull the bar down towards your chest. Top of the movement is when your chin passes over the bar or ideally when your chest touches the bar. Lower yourself down until your elbows are locked out. If you are performing multiple reps try to keep your shoulders active, pulling them down towards your hips.