I am going to build a new board for Frostgrave (which I can also use for Donnybrook and Mordheim). The recommended game size is 36x36", but I am going to make a 40x30" board which is nearly equal in square inches. The reason for this is I can use a single 1/2" piece of foam core as the base.
When starting any kind of project like this, I always start with a sketch. It doesn't have to be perfectly to scale - the main goal is to capture the feel I want for the design. In this case, it is a multi-level street with lots of staircases, while leaving plenty of room for buildings, ruins, etc.
After I have a concept I like, I make a scale model in Google SketchUp (free 3D software). I prefer to render my designs in a sketch style (they have photo texture fills, etc. but they always strike me as very 1980's Atari, and aren't really necessary for what I need).
There will be much more detail, but this helps me get the frame built. Goggle SketchUp has a measuring tape tool that lets me determine exactly how long each wall section needs to be, the height of stair cases, etc. I can even print out perfect side views so I can cut out the outer walls from a single piece of foam core. There will certainly be changes once I start cutting foam core, but without these diagrams it would be much more difficult to make this concept reality.
Assuming this goes well, I will probably build a second board as Mordheim and Donnybrook work better on 48x48" (mine would then be 40x60").
It may be a few weeks before I'm able to show much progress with the holidays looming, but I'll post more pics as I get on!