For all my aquascapes I come prepared with a general idea of how the shape of the rock will best fit the size and shape of the aquarium. I then pull all the rock (if it is already in there), fit in my base pieces, and then interlock the rocks together making sure they are as stable as possible. Once I have all the rock in place I'll use some underwater epoxy to secure it. Easy, mess free, and the best thing is it will be easy to change if needed. HTH
-Diana