Stand beneath a pull up bar.
Hold onto the bar with a wide underhand grip and hang from your arms.
Cross your legs and keep your knees wide.
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.
Try to avoid generating momentum from your legs throughout this exercise.