Most are attached to rocks. You can put the rock with the frag onto existing liverock and watch the zoanthids grow over other rocks (encrusting, except were are talking about softies). I always just epoxy in the rock with the frags on it or wedge it into a space within my aquascape.
Now, if you are getting frags from friends, then sometimes they will slice off some zoanthids and you will have to attach them to a rock via super glue gel (or you can put them in a tupperware container or something that block flow and allow the zoanthid to naturally adhere to the rock). If you are using the gel, I would go EXTRA light, as I have seen it kill zoanthids (the guys at logical reef can verify).
I will try to dig up a good zoanthid thread...
Take er easy
Scott T.