Work with a vocal coach to learn standard singing techniques before attempting screamo singing. The vocal coach will help you to find the singing voice that you are most comfortable with and teach you proper breathing and endurance for your singing. The coach will also teach you a number of warm-up and breathing exercises you can do before any singing session to prepare your vocal cords.
Practice singing in your natural singing voice. As you become more comfortable singing, begin experimenting with increases in your volume for short periods of time. Gradually increase the volume and duration of these periods, but avoid outright screaming.
Begin drinking warm water before practices and at regular intervals during your practice sessions. The water will keep your throat from becoming dry and the warmth will keep your vocal cords loose.
Practice making deep growling sounds that resonate from the back of your throat just below the sinuses. Be careful not to make the sounds in your throat, as you don't want to use the majority of your vocal cords for this; focus on keeping the sound coming from the upper portion of your throat and your sinuses.
Alternate between practicing your singing at an increased volume and practicing the growling sounds from the back of your throat. As you become more comfortable with both, gradually start to mix the two so that you can begin to articulate the "growling" into words.
Adjust your practice sessions once you have started articulating the "growling" into words so that you begin with 30 to 40 minutes of standard singing and then do 10 minutes of breathing exercises. Practice the "growling" singing for 10 to 20 minutes once you have finished the first two parts of the session.
Continue practicing the "growling" singing until you can do it for longer periods of time. Gradually increase the practice time that you spend on refining it and begin adding more volume to it as well. This will give you the "screaming" sound that screamo is known for, but will keep strain off of your vocal cords.
Practice regularly, always beginning with warm-up exercises that you learned from your vocal coach and moving on to standard singing to loosen your vocal cords before you begin practicing your screamo singing.