When did this thread appear and why didn't I see it??? Aaaaaaaanyway, it's more of a talent you're born with, but upon saying that, I managed to pick it up by randomly growling whenever I felt like it. You need a deep voice in the first place, that can help. If you're talking about Hardcore screams, though, you just have to inhale really quickly, through your vocal chords. Trial and error, mainly.