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Melissa Cross, aka Queen of Scream, is an experienced instructor who has worked with many of the top names in metal and other hardcore styles and she has an instructional DVD that you can order from Amazon here
I've heard good things about The Zen of Screaming series, but haven't tried it out myself. Melissa Cross is seemingly an expert in metal screaming technique. Otherwise, there are plenty of screaming tutorials on Youtube for free. Just remember, breathing technique is extremely important when screaming, and although some discomfort can be expected when starting, pain indicates that you are doing something wrong.
You might check out The Zen of Screaming series. I've only seen the first one and as the reviews point out it's a little thin on the actual screaming/growling technique but more on general singing techniques. It looks like the second DVD has more metal-focused material, but I don't have any firsthand experience with it.
Parts 1 & 2 are available on Amazon (confusingly, selecting the "with CD" option means Part 1, without means Part 2) :
(the original Zen of Screaming DVD is the best)
My friend used this to learn proper technique and she said it was super useful