First off you need to pick a technique.
From what i have experience in i can help you with two types:
False vocal chord, and phlem talking.
I use false vocal chords and it works fine for me but its a little painful at first but your false vocal chords with strengthen and you'll be good.
The way it works, there are two sets of vocal chords in the back of the throat, your vocal chords, and your false vocal chords.
Vocal chords are used for singing and talking whereas your false chords are used for coughing.
To use your false vocal chords for growling you start by just clearing your throat, this is the feeling you'll get while your growling.Now try to clear your throat and extend it for a while.
This is how the growl is formed from there you just have to work with that cough type thing for a while, then to further enhance the sound work with your mouth. For a deeper sound form an O with your mouth for a higher sound widen or open your mouth.
The other style, phlem talking, is more simply but (in my opinion) doesn't sound as good.In this technique you simply
gather phlem(stuff in the back of you throat when you drink milk)
or saliva and talk or sing with it. It creates a raspy sounding growl type sound.
Hope this helped.