The most common these days is a "tactile" approach where one person uses sign language and the deaf-blind person holds their hands to "listen". There's research going on to make American Sign Language more streamlined to support this sort of interaction, but while that's doing on, in the US at least, it's common to have people just sign ASL. https://www.youtube.com/watch?v=9GrK3P15TYU

Blindness is a spectrum though so it's possible that some deaf-blind people will have some sort of ability to see (usually takes the form of tunnel vision). In that case, they can still communicate similar to deaf individuals but it's usually modified so that the hands remain much much closer to the face (stay within view of what they can see).

As the other commenter mentions, tactile signing is the most common. There can be differences between that and the normal sign language used in the country. When I've communicated with deafblind individuals using tactile signing, I have normally just used the normal sign language for Ireland (Irish Sign Language) while they held my hands. It's a highly personal (and can be emotional) form of communication.