Masking is not any single technique, but a huge tool box or its own, with many possibilities. There is tons of general literature on the subject pertaining to color film if one is persistent in web searching, less on black and white technique, and nearly all of it now out of date with respect to current masking film options and suppliers of the right kind of equipment to do it efficiently.
Fortunately, contributions to the subject have occurred since this ancient thread began. But even by then, sooooo many specifics had already changed. So if anyone is interested in getting back into that game, it helps to be a specific as possible about your own anticipated use for it, as well as what specific kind of film(s) you are shooting relative to that, your type of enlarging system, etc. If you don't already have a full glass carrier (glass both sides) for your enlarger, that's the first thing you should get, or you'll be wasting your time.