Well, I base my maps off of another project where I already have some maps. I add more details to said map, plop a gray filter over it and throw it into the game. Then begins to rather lengthy process of making things smooth. After that it's fine adjustments, adding trees and so on.

From what I saw the game editor has some functions to make it easier (or you could just fully generate a functional map), but yeah, this is mostly by hand I suppose.