Say any word in the lowest tone you can give out and repeat it 10 to 15 times. Now add a growling scream to your voice when saying the word to create the metalcore sound. Pay attention to how it feels in your throat when singing this way to familiarize yourself with how your throat will feel when singing metalcore.
Read a sentence or a paragraph using the raspy metalcore scream you developed. Repeat this process until your voice becomes accustomed to pronouncing words in your metalcore tone. Constant practice is the key to sharpening your metalcore screams.
Scream loudly, to give your throat a raspier sound. Sing metalcore songs from your favorite bands as a standard against which to practice. This type of practice will strain your vocal chords and make your voice sound raspier. You can soothe your throat by drinking a nonlactose hot drink during your exercises.
Say "Ahhh" without using much air and add the growling-like scream to the word. Make sure to use as little air as possible. A traditional singer fills the diaphragm to sing, but a metalcore vocalist avoids the diaphragm so the screams emanate from the throat rather than from the diaphragm. This strains your vocal chords and makes your voice sound scratchier.