A frag can be mounted by at least four methods:

1. Just stuff the frag in a rock crevice - take the frag and just wedge it between two rocks or a small hole. This is not a recommended method as water currents snails and perhaps hermit crabs will push the frag over. This can stress the coral and increase the amount of time needed to encrust a new base for the coral.
2. Attach the frag with CrazyGlue Gel - this is a very popular method and some companies even sell "Reef Glue" which is nothing more than a Crazy Glue Gel type product with a different label but 5 times the price. For best results remove the rock from the water, pat the area dry with a paper towel and place a large glob of gel glue on the dried spot. Allow the gel to set for 45 seconds or so. Pat dry the frag and insert it into the gel. Hold for 30 seconds. If the frag is not stable hold a little longer or add some more gel.
3. Use Marine Epoxy Putty - not everyone will have success with the Gel Glues. I rather use a 2-part marine epoxy putty commonly found at hardware stores. And of course you could always get the "Aquastick" which is just normal epoxy putty repacked at a very high price. It is common for epoxy to be in a stick form with green and white colors. If you find epoxy using different colors be cautious of it. Cut off a thin sliver of epoxy and knead per directions on package. Epoxy is not a glue, thus it is not sticky. You need a crevice that epoxy can be stuck into to help anchor the epoxy in place. Push the frag into the epoxy and lightly push the epoxy around the frag as pictured here. As a side note, some people report that epoxy makes their skimmers go nuts and overflow. However, it seems to make mine go flat (no foam) for a few hours.
4. Attach with fishing line - frags from the Solomon Islands seem to always use this method. You usually need a frag with branches in order to use this method. But I find it too time consuming and a coral is too delicate to be trying to make knots on.

Whatever method you decide to use, keep the coral in an area of strong water flow as most corals will slime up after being propagated. This slime needs to be blown off to prevent the coral from being suffocated.