I got this. First off, the singer has to have just hit puberty. That's a must. He has to dress "homoish" and have a "gay" hair cut. Next insert a butt plug in his rectum and cover the vocal booth with pictures of male on male porn.
For best effect, have another dude in the vocal booth with him. He should remain quite, his job is to give the singer in question a hand job. Eventually he should switch to a blow job, respectively. Right at the moment when the singer starts to reach climax you burn the fucker with a hot iron, this is the real secrete. Rinse, repeat and record. Perfect "screamo" sound every time.
Radio shack microphones are known to capture this technique the best.
Disclaimer: I personally do not have experience using this method. I'm only paraphrasing what I read in "The idiots guide to screamo vocals"
Good luck and welcome to the forum.