Hmm, hard to explain over the internet how to do it, exactly... but first things first, you must understand that your voice is your instrument, and you only get one of them. You must treat it like you would treat a custom guitar signed by both Alexi Laiho and Randy Rhoads (
). So try to avoid doing anything that would harm your voice if you want to use it (these are just examples that destroy my voice: anything you smoke and inhale, spicy food, coughing a lot, talking a lot, etc.). Also, drink lots and lots of water... and I mean a lot if you want to growl! Yogurt was suggested, I noticed, and this is an excellent idea as well.
Okay, I'll try to explain about growling. All you need to do is growl like a dog, basically. Unfortunately, human vocal cords are not the best suited for making a growl, so when you start, you will sound awful and will most likely have very little range (esp. for higher pitches, unless you are previously acquainted with singing). And besides a lot of time and effort, there's only one other option that makes it sound better: pushing with your 'stomach' to get a beefier sound or higher pitch (I say stomach but it's actually various abdominal muscles, 'stomach' is just easier to say). This is a bad idea, because it rips and scars your vocal cords easily. The smart thing to do is practice until you can comfortably growl a few songs out (without any help from your abdomen... hey that's a way better word than stomach), and only push for the parts that it really helps, like screams. If you do that, you will be singing like Alexi does (sorta). The reason his growls are so effortless is because he so seldom needs to push with his abdomen. Of course you will have to practice a very long time to be able to sing at the pitches he does with that much force.
I almost forgot, but warming up, esp. for growling, is really fucking important. Even if you're only gonna practice for an hour or a half hour, warming up for a similar amount of time is a really smart idea. If you go from 0 to 100% instantly, your vocal cords will clench up and it becomes very easy to damage them. If you ease yourself into singing every time, your voice will become more elastic, durable and powerful far quicker than you expect...
Always remember that singing, no matter if it's screamed or growled or not, should feel
good... if it hurts, you're either doing something wong or trying to hard. It should flow from your mouth as smoothly as if you were talking. Just practice and explore your voice, and don't try to imitate other singers too much. Your voice will find it's own grooves to sing with and it's own comfortable range. Trying to sound like your fav singer is fun, but it's nothing compared to the fulfillment and fun you get out of your own style when you begin to refine it and explore your voice's potential.
Well I've outlined everything you need to know about growling, I think. But I can't stress enough that if your voice hurts from singing, stop immeidately, and try something easier. If you destroy your voice you can't buy a new one (well not yet at least). Hopefully this will help you.
The rest will come as it goes 
