Place a large resistance band securely around a pull-up bar. A band with more resistance will provide you with more assistance/momentum to pull yourself up.
Stand on a stable object (a bench will work), and grip the pull-up bar. With one hand, place the band around the arch of your shoe. Fully extend the banded leg.
With a neutral spine and your abs engaged, pull yourself up. The band will provide you with momentum to lift your body up. Lower back down to the starting position. This counts as one rep.