I don't think what CJWall said will be able to get you what you're going for - there's a reason we don't hear 'inward singing' featured anywhere. You need to be able to move air with your diaphragm (breath control is very important in any singing, some just know how to exercise it better and hit 'that spot' quickly and easily) and, to put it most intuitively, shove your voice back down your throat so that you feel like you're using your chest and not your throat to produce sound. Zen of Screaming is helpful to know what parts of your voice are there for, but practice is really the best approach - it's not that far removed from what you'd consider 'normal', if anything it's just using the same approach to your speaking as bears use when roaring, and you should be able to do it at a level close to or above your normal speaking voice... that 'silent scream' shit will make it very easy to throw your voice out without getting good results when you can, and if you want to be successful you can't just have it as a 'trick' - you should be able to order a pizza with your growl as easily as you would with your speaking voice.
Jeff