Stand beneath a pull up bar.
Attach each end of a long length of a resistance band to the bar, so that it hangs in a loop to waist height.
Stretch the resistance band toward the floor and place one foot inside the loop.
Hold onto the bar with a narrow underhand grip and hang from your arms, with your knees resting on the resistance band.
Pull your shoulder blades back and down.
When ready, pull down and back through both arms evenly and ascend throughout the full range of motion until the bar touches your chest.
Control your return to the start position, and repeat.
You should feel the resistance band assisting your movement through the upward phase of this exercise.