be sure to drink lots of water whilst screaming. I can say from experience that vocal injuries suck major balls.
as far as technique is concerned, i would start by tensing up every muscle involved with singing and start screaming. experiment with the shape of your mouth as well as the space inside your mouth and upper throat. Get the screaming 'tone' you want without the pitch and then when you master screaming, slowly add the pitch and be sure to get the right balance of growl and pitch. For practice, try growling different vowel sounds (ah eh ee aw oo) as well as your dipthongs (combinations of two 'pure' vowel sounds) (ay (eh+ee), i (ah+ee), oh (aw+oo), etc.).
yeah, that's about all I have. I'm a classically trained vocalist, so I don't do a whole lot of growling, but if you'd like anything more on vocal tecnique, just drop a PM.
--John