1. Use the back of your throat as if you're gargling with water, while buzzing your embouchure.
2. Articulate with the back of your tongue like a 'K' or 'G' sound. While you are playing a sustained G above the staff use you tongue back to slightly press against you mouthpiece while keeping the sides open. This should create a slightly fuzzy sound. Continue using this articulation while increasing and decreasing range to learn more control over growl articulation.