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.