hi Mr Genvin,
to be a good 'growler', or to simply produce the desired
'death grunt' effect, You have to have a specific body type.
If You want that deep, orotund Amon Amarth-type sound, You have
to be LARGE. I mean lung capacity is important. You can't produce
that kind of sound if You are not large enough.
If You meet the bodily 'requirements', practise is the key.
Start listening to Your favorite band, download their lyrics (I suggest
darklyrics.com) and 'sing' along with the song. Don't worry if it
sounds terrible first. It is important that You never strain Your vocal
chords too much or for too long, it won't help, and it can cause serious
pain. Second, drink much water, or preferably beer (but don't get drunk
,
because it relaxes the vocal chords, like right after waking up. A relaxed
chord is likely to produce a deeper sound, because it's somewhat longer.
MUSICisLIFE is right, while screaming, the sound must come from your guts.
After a few week's practise, Your own, signature voice will start to evolve. Be satisfied with it, You can't be the same as Your fav.
If You're truly disgruntled, try using some voice modeling processor; that way, You can create some ghoulishly distorted freaky sounds (like in Burzum). But personally I think the best if You can produce the growl without any high-tech help.
I hope this helps You somewhat,
Good Luck