Okay, then.
A scream is literally just a yell onto pitch, with rasp. Once you learn how to yell onto pitch, you add the rasp. The rasp is(of course) the tricky part. You need to make sure that your rasp is being created lower in your throat, as opposed to your throat being closed up top. Just make sure that your throat isn't closed. If you can't tell, you're basically screwed.
When you're practising getting rasp properly, only practise for 5 minutes at a time. Wait a couple hours and then try again. I say this because you will be doing it the wrong way at first, and doing it for any more than 5 minutes is going to tear and scrape your larynx up really bad.
Screaming is going to hurt in the throat for the first, I'd say, two weeks. This is because the pressure is much higher when you're screaming, rubbing your throat raw. I bolded throat, so you know that it's throat, and not vocal corRAB. You don't want your vocal corRAB hurting. A lot of people will tell you that you must not have any pain or discomfort in your corRAB when you start screaming. But the truth is, if you don't sing at all, and you suddenly start screaming, you're asking a fair bit more of your corRAB to hold notes when you're yelling. So you may notice a bit of problems(ie. voice sounRAB like crap, swollen corRAB) when you first start out, because of overuse. So make sure you warm up your corRAB, and make sure you exercise them just like you would any muscle.