1. - Open the terrain editor.
2. - Menu bar top screen - settings - choose map settings. Change it to 64.
3. - Click the elevations tab and choose elevation which is default 1000, you can make it lower, it's in feet.
4. - Find the brush size slider at the bottom push it all the way up, now your red dot is 6 miles diameter.
5. - Click in the center of your screen and you will have a big circle of land appear.
The tools under the elevation tab are how you create and carve land. Select one and your mouse is your tool then learn to carve up your 6 mile diameter dot of land. Make it taller, wider, carve gullys and canyons make mountains and so forth. The smallest unit of polygon is 660ft so your structures will have minimum size limitations. You will have to spend time learning them. Or you can always use Artik's program which pulls in GEO maps and creates importable heightmaps for the terrain editor. You can the link at the top sticky for this forum. In the end you will still need to know how to use the terrain editor to ultimately get you work published.
You have seen all three of my terrains and when I created the first one it was to learn the terrain editor. I pushed buttons a lot to see what would happened. Two years later of constant MA terrain building, you have probably seen riftval. It will take time and that still does not include the theory of terrain design for the low population MA these days. That is a lot of lurking and watching\listening to the customers along with researching the competition and some professional sites for level\terrain design. The paying customer is what matters if you are being honest about the endevor, not your personal dreams of an MA utopia.