One of the key things about good terrain building is preparation.
You need to know what the game system is, what mechanisms might come into effect, how figures are placed, what the setting is. Does the system allow figures to climb on buildings, or do they just run around on the one level? Are there rules for 'full cover' or 'partial cover'? Does the terrain need to accomodate different sizes of base?
For this first project, I'm just making a simple rectangular box...okay, maybe it's not just a simple box, it's a figure carry case and it's retractable, so it needs a degree of sturdiness and versatility. But sturdiness and versatility are generally useful to keep in mind with any terrain project. Sturdiness because you want this stuff to last, you don't want to have to make new terrain for every game played. Versatility because you want to be able to mix and match the terrain in interesting ways.
Once I get around to finishing off the series on geomorphs, I'll do a quick run showing how to combine terrain concepts and geomorph concepts to develop modular game boards.
For the moment though, a box that functions as a mobile peacekeeper station.
The first thing I do is draw up the terrain piece at full scale. Floor plan, elevation, and interesting angles that might make the piece a bit more interesting. Points of interest are good, but you need to make sure they add more in aestethic value than they take away in playability.
The next thing I do is measure up the board I'll be making the project from. If I weren't planning to use this piece as a storage box, I'd probably make the piece from thick card or foamcore board. Instead I'll be using MDF.
The next thing I do is cut those shapes out.
That's made quicker with power tools, but can be done just as easily by hand. Just make sure you leave a gap between the various parts, because a saw blade has a width of its own. I've seen a few projects go awry because someone hadn't considered that simple width of the blade. I like to make at least a 5mm gap between components.
Once the pieces have been cut out, make sure they are planed or sanded down fairly accurately. Even if you are making terrain that is designed to look decrepit and abused, it needs to be sturdy at it's core structure. The decay and abuse can be done with weathering techniques once the base piece has been created.
You might also note that I like to label all my pieces for easy identification.
Like I said at the beginning, preparation is key.