Fix a TRX suspension trainer around a bar and sit directly under it on the floor, legs extended straight in front of you; grab both handles, arms straight (a). Without leaning back, bend your elbows and pull your upper chest toward the handles (b).
Great article on pull-ups, useful for beginners (although it calls them chin-ups). Overhand grip is pull-ups .underhand is chin-ups (where I come from! One of the best compound exercises for b (Fitness For Beginners Bodybuilding)
Loop a resistance band around a chinup bar; place your knee in the loop and grab the bar with an underhand grip, arms completely straight (a). Pull your chest toward the bar (b). Pause, then slowly return to start. That's one rep.