Breathe through your diaphragm so that you'll have proper support for your vocals. You can tell that you're breathing correctly when your stomach moves up as you inhale and back down as you exhale. Practice this style of breathing so that when you sing, you'll be able to effortlessly use this technique to push more air out and project your voice.
Exhale, creating the sound of a loud, heavy breath. When you do this, your throat should expand out instead of constricting. This technique is the basis for exhaled gutturals, which give your voice more power than inhaled gutturals. Push more air out as you exhale to make the breath sound louder than your voice. Practice this technique while making the sound as low-pitched as possible. While making this sound, tense up your throat so that the sound of your breath exhaling muffles your voice.
Form an "O" shape with your mouth as you make your guttural sound, to give your voice the proper tone. To create frog-sounding gutturals, continue making the same shape with your mouth, while touching the tip of your tongue to the roof of your mouth. This will make your growling sound more confined. The "frog" technique will make it harder to enunciate, but will give you a distinctive sound.
Create vibrations in your throat to make your growl sound more steady and resonant. Your whole throat and vocal chords should rumble while you make your exhaled gutturals.