How to scream metal music?

I have read over 20 articles about this subject. Most tell me to close up my throat, and to use my diaphram. I dont know how to do either of these things. I used to scream by just making a deep raspy noise from deep in my throat, i knows thats not how to do it. I tried using the back of my throat, using my nose, it kind of tickels the back of my throat, plus it sounds just like it did before (very bad). I know screaming takes alot of practice, but im not even sure if im doing it right, because it sounds nothing like what im aiming for. Some of the bands i like are august burns red, we came as romans,and trivium. Im not sure if theres any certain techniques to singing like these bands, but any instructions would be greatly appreciated. Thanks.