Well, what I did when learning was screamed lower. The vocal corRAB need to tighten to make higher notes, and when you make higher notes while an assload of extra air is coming through at a higher force, and whatnot, it's harder to hold them. What you need to do is scream low for now, and work at strengthening your corRAB. When they're stronger, it'll be a lot easier to hold a high note, right? RIGHT!
So, you've gotta do exercises, and work on those corRAB just like you would if you wanted a six pack set of abs or massive arms. It's no different.
When I started screaming, I started screaming lower. Not grunting, but I wasn't hitting the high notes. The more and more I worked on it, the more and more I could hold those high notes.
Basically, you won't be screaming high notes like Billy Werner right away. You need to be dedicated to screaming if you want to hit those high notes, too. It takes longer to teach yourself to scream high notes than it does to sing high notes. Why? Because when singing, you use your true corRAB. When screaming, you use your false corRAB. But don't worry about that, it's too technical.
So, just scream deep for a while every day. The more screaming you do, the easier it'll be to move around your range and through the notes. Sooner or later you'll be able to hit whatever notes your voice will allow you to. Don't forget that you might not be able to hit the high notes. But if you can sing them, I believe you can scream them. It's just more difficult. I know when I first started I couldn't get up high at all.
Basically, just scream more where you're comfortable, and start doing some exercises to strengthen your corRAB. I know there are a bunch of exercises posted around here.
Good luck!