Work from your lowest level, water level / river bank / ocean beach. Make these depressions in the surface and build up around them, packaging foam is easy to work with but a little messy. Paper Mache is also good but again a bit messy. At this stage don't worry about smoothing out edges as you may redo parts as you go.
Work out your ski slopes, mountain backdrops etc so overall height is known. Make sure you will have access to wiring etc once you have it completed. If possible make the scenery modular so you can pack up and reuse the following years.
Prepare space for roads, railway etc with street lighting and then make areas for your buildings to go. Make a base template from cardboard or similar and write a description and the position of the power input. That way you can spread out your building pretty accurately without getting them out of the boxes. TBC
