dab the plug with a cloth (or even a q-tip) to dry the part where the glue will go, put a drop or two on that glue on, stick the frag on there good. you probably want to leave it out of water for a minute (don't blow on it). after a minute, rinse off the frag and plug with saltwater (to clear off any residue). the rinsing will also help the glue harden...
i've put a freshly glued frag back in the tank without rinsing it, and some transparent material came off - probably glue residue = bad. after rinsing, its ok to go back in the tank. i usually turn the frag upside down (while submersed) to make sure its glued right (and not barely hanging on).