Thanks, arsheesh for the welcome and advice

I took a look at the continent style tutorial on your advice (and very good it is too - no wonder it's so highly regarded) but I'm currently doing OK for region or country scale maps (after doing some Google maps patchwork).

The part I'm having trouble with the most is the more intimate scale maps - maps of individual outposts, the interiors of shuttles, battlefields, etc. I've been dabbling with a rough shuttle interior using SketchUp, which I might see if I can do a paint-over for some much needed grunge.