Hi! :)
For most of my layouts I´ve been either buying ready-made walls and platforms, or been engraving in plaster on a wooden plate.
Now however, I´ve gone over to making all engraving in Styrofoam, finishing with a layer of plaster and then Acrylic paint.
The colours can be varied to fit whatever scenario you´re going for, but the process stays more or less the same.
https://www.youtube.com/watch?v=B0zzVtGXRK8 (https://www.youtube.com/watch?v=B0zzVtGXRK8)