sc_rebel: The how part of mods - there isn't really a straightforward answer -or one size fits all type of answer. For me, a lot of it depends upon how comfortable one is with an image editing program and working with layers. If all of that sounded Greek, then -well it might be a little tough to explain.
At any rate- as far as the maps go, that would all be done to the bitmap files in the Map folder of any given title (this applies to pretty much all John Tiller designs -or at least all that I have had a chance to see (I haven't had a chance to see any Modern Air Power -so it might not work the same way there). Basically you change the images in the hexes, hexside features, or pretty much anything you want to take a shot at.
I know they have some mods over at the Gamesquad.com download area -however I think that largely those are the same ones that have been available on Brett Schulte's site (which I don't have the link handy -but I imagine someone in the club would as this question comes up from time to time).
I've done mod work myself for the 2d maps in a number of series - I use a version of my own -although I never put together an actual mod package yet.
I should add that in that draft version the houses were a little on the big side -so I have done some work on that - some of it has worked out, while others -not so much
-but you can get the idea.I was working on some water hexes too - I had a fairly nice set with some texture -but then decided that the Mississippi might look a little better in a less blue shade. Although it is a bit of a challenge when you have to work with an entire hexagon.
Anyways I figure I may get time to maybe get one done and downloadable in 2011.
Edited to add that the first screen shot might be a little deceiving in that I am pretty sure it is from one of the Ozarks maps- possibly Pea Ridge so the elevation changes are a little all over the place. I don't actually have one uploaded anywhere of say, an actual battle. I do have them somewhere though.
This one below is maybe a little more representative.
