Anal fisting is insertion of (most of) a hand into the rectum. (Vaginal fisting see here.)
As with any other anal dilation methods, it will take time for the receving partner to relax and stretch enough to accept something as large as a human fist. Butt plugs and anal sex are good ways to start. Especially butt plugs can be increased in size for stretching.
For lubrication, especially for beginners, oil based lubes are recommended because they last longer and easily cover the whole hand.