Practice singing as much as possible, and then begin to play around with pitch and tone, creating your own scream voice while singing. Remember that shouting or screaming uses a completely different part of your vocal chords, so you must practice reaching a scream sound while actually singing. Once you learn the technique, you will recognize that scream singing is no louder than normal singing and will never reach the volume of an actual scream or shout. You have a microphone for volume.
Warm up well before practice or performance to protect your vocal chords. Practice singing scales, beginning in a low voice and working louder. Like any muscle of the body, starting off with cold vocal chords is a great way to hurt yourself.
Prepare for scream singing practice or performance by avoiding all dairy products for at least one day. Dairy products cause mucus to build up around the vocal chords, forcing you to strain to sing. You can also improve the ease of singing by drinking plenty of water and gargling a bit of salt water before warm-up.