In order to train your voice to perform screamo singing, you'll first need to master the art of singing with your natural voice. This is to get your vocal cords in good shape. Vocal cords are soft tissues controlled by your larynx muscles. In order to better control your vocal cords, you need to practice vocal exercises to more accurately regulate air flow, which is what actually creates the sounds of speech. If you don't get your vocal cords used to different muscle movements and breathing techniques, you run the risk of damaging them. You can find vocal exercises online or by speaking with a vocal coach.
Although many vocal coaches practice traditional singing (classical and pop singing), there are some vocal coaches available who can help you enhance your screamo singing. Call around and ask potential vocal coaches if they can help you with screamo singing. It's important to find the right vocal coach so that you're learning the right skills. Your coach will have to help you find your false vocal cords, which involves using the back of your throat and pushing air out with your abs. You also need to learn how to breathe properly.
Learn various vocal warm-ups used in traditional singing. These include running your voice up and down scales, practicing breathing techniques, practicing dynamics (the loudness and softness of the notes you hit) and control. These help you to tame your voice, allowing you to better control the sounds you make. The more control you have of your vocal cords, the better a singer you'll be. Make sure to perform your warm-ups for several minutes (up to a half hour) before screamo singing.
Avoid dairy products, as these tend to muffle the vocal passage and cause mucus to overproduce. Excess mucus will force you to clear your throat, which causes the vocal cords to become inflamed and irritated. Because vocal cords are soft tissue, any irritation can dramatically affect them, causing your voice to be scratchy and uncontrolled, which will make it more difficult to hit the right notes. It's also a good idea to avoid alcohol and drugs. These also inflame and irritate your vocal cords, and will make screamo singing much more difficult than it has to be. Drink plenty of warm water, both throughout the day as well as right before you sing. Because vocal cords vibrate and rub each other, lubrication will help them function more properly.