Start practicing regular singing. Sing some classic or alternative rock such as Stone Temple Pilots to help strengthen your vocal cords. Practice as much as you can for a month before diving into the screaming and wicked growls.
Drink some lukewarm tea, water or black coffee about one hour before you start practicing. Drinking warm fluids helps to relax your vocal cords. Find your vocal range by finding the middle "C" on a piano or guitar. Play your instrument up and down the scales. Sing along with every note until you can't go any farther.
Warm up your vocal cords by singing the typical "do," "re," "mi" or humming. Focus on your nasal region above your throat. This is where you want to produce the sound. Fill your diaphragm with air. Breathe into your stomach and not your chest. Push air up into your nasal area and create a growling sound. Practice making a growling sound for a few minuets.
Force a sound out of your diaphragm that imitates a dog's bark. Try inserting words as your push out air. Practice this until you feel that it's perfect.
Place your tongue in the lower part of your mouth to produce a low-pitched scream. Open your mouth as wide as you can and let out a low scream using your diaphragm and nasal region. Produce a high-pitched scream by raising your tongue so that the sound hits the roof of your mouth. Breath deep into your diaphragm and scream, pushing air through the nasal region. Repeat this a couple times while changing the pitch.
Start practicing different songs. Start with moderate classic rock with slow vocals and end with some hardcore, fast-paced death metal. Drink 8 oz. of water after you finish practicing.