You honestly can't make your voice raspy without extreme physical modification. It is natural to have the voice you have now, and it can be harmful to your singing voice if you go through routines to make it raspy. Raspy voices are a blessing, but when forced through an uncomfortable setting makes your throat hurt and can harm your vocal cords.
Paul McCartney wanted to do this for the song "Oh Darling" on the Abbey Road album, so he spent a week or two before recording screaming the song at the top of his lungs on a daily basis in order to throw out his voice enough to make it raspy for the recording. Supposedly, John Lennon was angry with Paul for not letting him sing that song on the album.