Depending on the frag
there are different methods.
For the more stoney corals you an use superglue GEL and put a good mound of it on the frag
and then put it in the water and give it a few small twists in a crevice of the rock you want to put it.
You could also use two part marine epoxy.
For corals that attach faster like the soft corals you could use a rubber band, a string or a little bit of wedding veil to hold the coral to the rock surface until it can attach.
Can you tell us what kind of frags
you have and we can suggest more specific procedures for you.