I guess I'll throw in my two cents.
When I growl, it isn't conversation volume; it is certainly louder. However, it is not nearly as loud as when I scream. It's hard to describe what goes on, but when I growl, it's almost like I'm singing an actual note, but really forcefully, and I'm closing my throat a lot when I do it. The back of my tongue tends to raise towards the back of my mouth, also. The lower the growl, the further back in the throat it is. It helps to have phlegm in your throat, too, but it's not necessary. I used to experience soreness in my throat after growling, but I never do anymore; at most, I'm a bit hoarse, and that only after I've been growling/screaming/singing a lot. I don't know whether the reason my throat doesn't hurt anymore is because it's gotten calloused, or because my technique has just improved. I think it's the latter. In any case, my clean voice hasn't suffered from growling (mind, I'm not good singer anyway), it's improved (not as a result, though). Actually, the main thing I have to worry about hurting is my fucking head. I send a lot of force when I growl or scream, and I suppose it sends blood to the head, and if I'm not careful, then I end up with a really bad headache. The first time I ever experienced this was one day when I was screaming along with Emperor. I realized that I was sounding quite a bit like Ihsahn, so I kept going... and then I stopped. And there it was -- the worst headache ever. So just be careful of that.
Of course, I'm no teacher or actual vocalist, but this is just how I do it. Hope it was at least somewhat helpful.
One more thing, you will have to practice it a lot. Don't get discouraged if you sound stupid for a while (obviously, to some people you will always sound stupid... or insane), but just keep going. You can basically feel your way around, and try different things and see how they sound. It's a matter of developing muscles that you don't use much, for one thing (I think). So good luck. \m/