If you can find a dead spot in the tank on the sand bed, let the xenia sit there for a couple of days. It will attach to the sand bed, then simply pick it up and glue the attached sand bed to a rock or frag disk. In my frag tank I will put a small bowl with media for my calcium reactor in it. Lay the xenia in that and in a couple of days there will be a couple of pieces attached to the xenia for easy gluing.
Good Luck