Ok.. I'll give you a lesson...
Step one... Clear your throat..
Imagine that you are trying to hint to some one by clearing you throat...
that sound that is made is the basis of the growl...
Now try and do it for longer periods... try doing it high and low, and (While no one is watching) make different shapes with your mouth and throat etc... try and experiment.. and see what kind of sounds you can get..
I'm better at more high pitched black metal screams... but these are hard to acheive, and actually took me a few years to master.
I start growling along to Fear Factory, in that style, but as my music tastes changed, my voice changed, and now I can sing along to my favorite Enslaved and Emperor songs.. But I still can't imitate Mikael.. and I'm no good at low growls.. I do them higher, and I sort of do it by growling from a whisper...
Tomorrow, while no-one's home.. I might try making a recording.. Tho, I'll sound like a retard...