The most common method is sandblasting, although this will produce a rough ground glass screen, it will be servicable..or many have used gease based valve grinding compounds normally found at auto repair shops, normally, using sandpaper is not to sucessfull. Another method is to make a table to hold the glass that randomly moves up down and in a occilating pattern then add metal ball bearings and leave it run for about 24 hours, of course you can stop by your local hobby shop and pick up some acid, like they use to acid etch patterns on wine glasses and then coat one side and then wash off and repeat until you get an even etch

Good luck on your project.

Dave Parker
Satin Snow Ground Glass