1. Practice singing higher. The higher you can sing, the higher you can scream.
2. Use everything to your advantage to create tone. This includes: The shape of your mouth, any gritty imperfections in your natural voice, and visualize yourself going higher.
3. Lube it! Use cool water to keep your throat tight and crisp.
4. Slowly start trying to scream from your throat. There is a fine line between under practice (where you will not get any better) and over pratice ( where you will blow your shit out). Your throat will get tougher and be able to handle more and more with time. Dont be affraid, you could sing wrong and even hurt yourself, screaming is just something new to your throat. BUT! IF YOU REACH A POINT TO WHERE YOUR HEAD HURTS AND YOUR VOICE CANT REACH THE HIGH NOTES ANYMORE, STOP!!! This has happend to me before, and I had to learn to scream all over again. So monitor yourself, okay.
5. Laslty, think of your favorite singer, and without screaming, try to mimic their voice perfeclty under you breath. And just like imagining making a basketball shot, bring that voice into a full scream.