I use a superglue gel (ethyl cyanoacrylate) to mount my frags. It will not leach any harmfull substance into the water which will harm fish.
As far as your question goes, I place it in the water as soon as possible after applying a good amount of superglue gel to the fragment. I then take the frag out of the water, apply another good "coating" of superglue gel, and place the frag in the water for a second time. I then the take the fragment and rub it around the rock or area I want it to be (of course, I rub it on the side containing the glue). After a few seconds the glue catches on the rockwork, and the fragment is mounted.
By the way, what are you planning on mounting? My method for glueing fragments was guided towards SPS corals.
Graham