If you need a how-to for singing death metal, then you just shouldn't sing death metal!

Just open up and let the chunks of blood and rotted flesh flow from your lungs, and try to vomit as many zombies and carcasses as possible. It also helps to thrash until you die from it. For added fun, you can take the John Tardy (Obituary) approach and forget about singing words all together and just growl gutteral sounds that occasionally sound like words. It's what the undead would do anyway.