You'll be needing a few tools in order to make landscape like that; such as Map Editor, IMG Tool, etc. I used to know a few links to tutorial about this kind of stuff but it's f*cked up (removed). I'll try something else (I'll send the link as soon as I find the correct one; I also need the tutorial, not urgent though).
By the way, why don't just make a call to the author of that mod and ask for a help. It's on the video somewhere, I think.