Often, if the frag has something already it is attached to, i.e. a plug or rubble, you can glue it in place IN the tank. If it is a unattached fragment. You need to attach it OUT of the water.
What I do when it needs a permanent home in the tank is to goop up a big ole gob
of SG gel on the rubble or plug it is attached too. Then place it where I want it. pressing down and twisting then hold it in place for about 30 seconds to let it set.