How to Work the Heavy Bag?
Just grab onto the handles and hope they don't break.
Good advice from Ancient One. You need a rhythmic start and once your rhythm sets in, you can work in building punches. Arm and stroke length punches, shoulder punches, short punches, jabs, hooks, etc.
In the very beginning start out slow and methodical. Set the bag at the right height for you. Mark the bag, with chalk targets, Position yourself in a "limber" / flexible stance. Start out slow and hit your targets in the same order repeatedly. As you gain confidence in your stance, usually the third day of practice, begin random fast punching. If you wish your bag time to be of any use, you must practice several hours a day, with many breaks.