I actually just ended up super gluing them to the cardboard after I traced where the building was going to sit. If I was to do this with plasticard, I would still be cutting them out and carving them. I'm interested in seeing how this effect works.
To add some more detail to the base, I glued plumbers sand around the edges, and added pieces of old sprue to give the building a more dilapidated look. I also glued some bit of sand, course basing material, and sprue to the tiles themselves to add a bit more texture to it.