So I'm a geography geek, so this is right up my alley. So first of all, not everyone needs a map (I just like them because). When converting an idea to a map, first think about the environment of that place. Is it a swamp, desert, mountain, basically which biome? Then look at maps of places with the same biome. For example, if you have a swampy area, but you're not sure how to make that work out, consider its surroundings. Is it coastal? Is there a river delta? Is it inland, and if so, are there mountains nearby?
Simply put, build it up one region at a time. And then, you can switch the regions around to fit your liking and such, like pieces of an interchangable puzzle.