Just minecraft Singleplayercommands stuff! XD )Worldedit)

All you have to do is select a weapon and type in /brush sphere 2 5

or this : /brush sphere <blockID> <Radius>

then just start clicking to build mountains, once you've done that type

/brush smooth 5

Then start clicking where the mountain area is, It will start to make it look more like a mountain instead of random spheres :)

So hopefully you understand that! XD