Stand on the steps of a chin-up machine or place a bench under a under a chin-up bar. Grasp bar with an underhand grip that is shoulder-width apart.
Next, hold yourself up so the the bar is at your chest and arms bent (you may need to jump up slightly to get to this position without pulling yourself up to it).
Next, begin negative chin-up by slowly lowering yourself down until your arms are straight. Try to lower yourself so that it takes more than 5 seconds till arms are straight.
Release grip, step back up, and repeat.