To growl you need to find you false vocals chords which I guess are somewhere slightly above the reals ones (the ones you use to speak). Try to sing like Louis Armstrong and you can find them. The growl is a mix of that rattling sound the false fold create, and a normal low voice made by the real vocals chords. To shape the growl you must tighten or loosen your throat. Start practicing very silently first and add volume only after you feel you've found a comfortable way to make the sound. It'll hurt a bit at first, I don't think there's a way to prevent it. If your throat is sore after practicing give it a rest enough to become normal again. It should be easier after that.