Get into a basic boxing stance: stand with your feet shoulder-width apart and your hands up by your face. Turn slightly side-on to the bag so that your left side is closer to it (assuming you are right-handed). This is the starting stance for all the boxing moves.
Jab your left fist straight out from your shoulder into the bag, then hit the bag with your right fist, turning your whole body into the straight punch. Complete the combo by jabbing your left fist into the bag again. Repeat this combo 20 times.