When you are thinking about creating a map you need to have an idea of whats going to be on it, what you will need, and how you will do it. It is best to start with a page of assets so that you know exactly what you need. You also should make a rough outline of the map so you have a decent idea of where everything will be. That way, you can go into the map editor and put everything where you want it and not have to leave unless you find an issue or decide you want something else. This helps you be extremely productive and saves you a ton of time. On your list of assets, you should have the following:
-Static objects needed -Road textures needed -Terrain textures needed -Sky textures needed -Decals needed -Anything else you plan on putting on your map I didn't list
Now that you have everything you need listed out, begin making everything. I recommend starting with the static objects, they are the hardest and if you can't make them, it would suck to have everything else done, and then you can't make what you need. Then I would do terrain textures, I would load up the map and test these before you actually start placing objects. Now make the road textures, sky textures, and make a heightmap. Now you can load up your map and easily look at your design, and start making it come to life.