Some guys use a tool (I have no idea what it's called, but it's like a pneumatic reciprocating punch). Others use a punch with a hammer, which is much slower. I'm in the latter camp, as I rarely stipple anything.
I take a steel punch (any size will do, as long as it has a failry thck shank), and I grind it to a point on 3 sides. It doesn't have to be very precise, as you are just looking to raise burrs. Then I harden the tip with a torch, heating it red hot, then quenching in oil.
Working on some scrap metal or a junker frame will give you a quick idea on how to do this. You just place your piece in a vise, place the punch tip on the metal, trying to keep it perpendicular to the surface, and start hammering. If you hold it above your work a little, you will get a larger burr. If you have a larger, heavier punch, you will have a larger burr. You just keep hammering away at the surface, letting hits overlap. Eventually, you will have it all covered. Also, realize that on the edges you will be prone to slippage, so a piece of scrap metal braced up against the side can act as a guide. I did not use one, but I would have if I had intentionally done this gun for someone else.