Stand in a staggered stance with one foot in front and your chest tall, fists up by your chin and your elbows against your ribs.
Extend your nondominant fist out to meet the boxing bag (if you're right handed, use your left arm; if you're left handed, use your right arm). Lean into your front leg as you punch.
Bring your fist back to your chin while simultaneously twisting at your torso and extending your rear arm (your dominant hand) through to a cross punch. Make sure you're using your entire body to lean into the punch and pivoting your back foot.
Return to your starting position. This counts as one rep.
Repeat these jab and cross punches for 30 seconds.