Find and listen to raspy black metal. You need to get a feel for the music, and for the vocal style that you're going to imitate. Bands like Venom -- said to be the original black metal band -- Angelcorpse or Dimmu Borgir are some good bands to start with. Learn some songs, and find ones that appeal to you that have the lower, raspy style of singing.
Practice breathing with your diaphragm rather than your chest. Rather than forcing air out through your throat with your chest, you need to bring the air from all the way down in your stomach. Flex your stomach as you breathe to get a feel for how deep you can pull your air.
Drink some warm milk or water to loosen your throat and protect your vocal cords. Dehydration can lead to strain and damage, and it's important not to strain your throat when practicing.
Practice singing in a raspy tone of voice along with the music (without the lyrics if you can find it that way). Flex your throat to hold the rasping tone and force the air up through your throat from your stomach. Pay attention to how your throat feels, so that if you start to strain you have time to recover.