I've been a vocalist since my HS freshmen year which was 7 years ago, now in college and still with it.
If you want to growl low, like really low, learn a false chord technique. If you want more of a scream or airy sounding growl learn vocal fry. False chord will still deepen your voice and if pushed to hard to often can cause development of the dreaded nodes on your vocal chords or false chords (located just above the vocal chords).
It's nigh impossible to safely learn false chord technique without first learning vocal fry. This is because you have to feel where the sound is coming from. If you are properly engaging false chords you will feel and potentially hear a faint electronicy buzz above your vocal chords.
It took me three years to get my fry to a place where it wasn't incredibly difficult to perform consistently. This is not a quick process and it will be quiet when you start out. Your goal is to find the fry register. This sound is made by vibrating your vocal chords in combination with intense diaphramatic pressure. First make the "grudge sound" an easy way to achieve this is to make a note and hold it, then slowly use less and less air until you hear that creaky sound you made to annoy people as a kid. Tighten that and push with your diaphragm. Half Lower your larynx and raise your soft pallet high... As high as you can. That's all I can really describe in text look at some vids on the tube, but ignore any vocal feud that sound not like crazy high pitched shrieks because the sound isn't coming from the soft pallet they are using their throats.