I use a combination of super glue, lots of epoxy (Aqua Mend) and when building unstable scapes I also use a fishing line.
Basically - the super glue makes the first contact points to temporarily hold the scape in place.
The epoxy is than used to wrap these contact points quite generously.
And finally, the fishing line - when the scape I built is a little wobbly and isn’t stable enough, I secure it by tying the fishing line to a higher location (I use the lighting mount) and going underneath the newly glued rock, sometimes in multiple places to firmly support it until the epoxy fully dries (a day, sometimes two).
It is advised to ask for another person for help, 4 arms are better than 2.
First few times feel somewhat complicated, but overtime you get used to it and it becomes easier to execute.