Brief summary of something I read on a "how to growl" webpage...
1. Learn to sing before attempting something like a growl.
2. Involve the diaphragm when singing and growling, instead of putting all of the stress on your vocal chords.
3. Practice a few times before attempting a really awesome growl.
4. Drink plenty of warm water.
In my case, I don't have the deep, bassy voice for an impressive, scary sounding growl. So my attempt turns out sounding more like a small to medium feline.
My cat has this high pitched meow that I can imitate perfectly. He then "talks" back to me. Usually, when he meows, he's wanting some more of his canned food.