Well If Its Your First Time I Suggest You Watch -- Feral Heart Map Making Tutorials - youtube.
Or A Simple Way To Explain -- Go To Map Maker -- World -- And There Are A Bunch Of Terrains To Choose From.
If you Make your own terrains DO NOT EDIT THE DEFAULT TERRAIN DX!!
You Copy The Default-Terrain, then you open it in either -- PhotoShop -- Gimp -- Or FireWorks.
The Colors You Use For It Is [[The Proper Colors "RBG"]] Blue, Red, Green -- Purple, Yellow.
Then you type in the name on the map maker Under ""Terrain Alpha Map"" [[Example -- Map01.png]]
Theres Your Map! If you need any more help just ask.