Halford once revealed the way to use screams in metal. This new-fangled screaming stuff that seems to have become so popular is akin, to my mind and ear, to the method of doing a kind of buzzaw riff on detuned giutars instead of doing solos, however, good vocal techniques are the same for whatever you want to do.
About ten centimetres - four inches - below your navel is a muscle called pyramidalis. This muscle is our friend, because it allows us to press the air in our lungs upwards. Start of by placing your hand there and try to find it, and then use it to just press upwards. When you push it hard, the rest of your stomach, including the legendary diaphragm, will start working too - but KEEP - CHEST - STILL - and - SHOULDERS - RELAXED. Just let pyramidalis and the diaphragm work as a bellow for you.
Once you've found that, you must practice how to get air in. This is done by relaxing. No kidding, when you open your mouth and relax throat and pyramidalis (to start with, exagerate a lot - really slouch) you'll find that it's easier to just take in the air than not.
Next comes the act of connecting your support. This means tensing pyramidalis, as described above, pushing up but don't let any air out. Then, when you start making sounds - don't do anything difficult to start with, it doesn't even have to be notes - concentrate on making sure that you're pressing the air upwards. This is where you get your force, your platform for using your throat. The vocal cords, and the sixty plus muscles around it, aren't designed for heavy work. They got one speciality, vibrating to create sound waves, so do not use them for anything else. Pyramidalis - and, in the long run, once you get the hang of it and relax enough, all the muscles around your waist - are designed for heavy work, so let them do it.
There are many exercies for getting this right, both for finding that support and using your vocal chords to create what's called a "free tone"... but this is the basics. Just ask away if you're interested in more.
Listening to Cannibal Corpse and cutting trees with a chainsaw, now that's metal
"He preferred the hard truth over his dearest illusion. That, is the heart of science."
- Carl Sagan
"Imagination is more important than intelligence" - Einstein