Stand up straight and take a deep breath and hold it for five seconds. Exhale and repeat the step again. This will prepare your diaphragm for the harsh vocals.
Practice by singing your favorite song or lyrics with a regular voice. Don't over strain your voice by practicing high notes.
Take a deep breath and exhale while tightening your throat. Take another deep breath, say some lyrics as you exhale and tighten your throat. You should feel a vibration in the upper part of your chest or shoulders.
Relax your throat and inhale for five seconds and "roar" as you exhale the breath out. Doing this will loosen and relax your vocal chords. Sing slowly at first and increase the deepness of your voice as you continue. Growl and exhale lyrics to add detail to words.