I've used several methods:
Super glue gel from the dollar store is an inexpensive cyanoacrylate glue which works well for mounting smaller frags to frag discs. This is useful for zoanthids too.
Using some epoxy putty with some of the glue on the downside pressed into the rock with some on the top side to hold the discs provides an easy way to put the frag where you want it in the aqua scape.
The epoxy putty puts off heat when it dries so try to avoid contact with the coral flesh when it's curing. Use it in small amounts at one time or it will cause excess foaming in the skimmer.
Sometimes, a zip tie through a hole in the rock provides a nice tie down spot.
For leathers, gluing a toothpick to a piece of rubble and piercing the coral with it works well for most. The coral will affic to the rubble ,the toothpick rots away or part can be snipped off Sometimes ,taking two small [pieces of rubble and sandwiching the coral works well.