I looked this up when my friend offered me a spot in a punk band. I didnt do it cause I sing not scream.
PRACTICE, EVEN IF YOU SUCK.
Make sure the noise comes from your nasal cavity or chest, not throat.
Drink water.
Warm up singing.
Make noises first, like Marge Simpson voice.
Have CONFIDENCE(why I cant scream)
When you've damaged your voice(we all do it) speak normal, dont shout dont whisper.
You can start a post-hardcore band which is the same instrument except littlscreaming
Wish I could link you.