This is a basic how to on laying down a foam riser and covering it with plaster cloth. This is just how I do it, there maybe better and other ways to do it.

The foam track bed has been glued down for the outer track, most of the track has been cut and is loose fitted. Time to try some trains! Sorry for the non-prototypical running, the idea was to test more than run, but it ended up being a very long session.