Rehearse singing in key with your clean singing voice with some of your favorite songs that have vocals in your range. Screamo entails the use of both clean vocals and more aggressive screaming. The better your clean vocals, the more expressive the entire song will sound.
Warm up your voice with a vocal exercise so that when you do scream, it does not damage your vocal chords. Many of the best screamers such as Randy Blythe from Lamb of God use "The Zen of Screaming" by Melissa Cross for direction.
Practice by yourself switching between melodic vocals and the more harsh screaming voice that you will be using in a band. Try to build up stamina while switching between the two voices. Use your diaphragm and breathr as much as possible, as screaming takes a lot of lung power.
Attempt to scream in different pitches to extend your range of screams. The more versatile you can be, the better you will be able to express yourself with your music and vocals. Try some screams higher than your usual range and lower.
Join or form a band that plays screamo and practice with the band to see how well you control the volume of each style of singing. Use microphone control and its distance from your mouth to make both vocal styles similar in volume for consistency.