How exactly do you bench bleed a master?

With a new master usually you get some plastic fittings and rubber hose that screw into the ports on the master. Take the tubes and loop them into the master fluid reservoir and using a Phillips screw driver pump the piston until it firms up and no air is being pushed. Usually only takes a few pumps. If you don't have the fittings you can get a couple pieces of short brake line that screw in and do the same thing.