I think making custom distant terrain is quite advanced, and you'll need to know how to mesh. There's a tutorial
here on modthesims but it does say basic meshing and texturing skills are required.
However, you could try recolouring, this is a lot simpler. If you search google there are several tutorials for this. An example of a world that uses recolored distant terrain is
Ziwa Bonde , which has recolored Egypt terrain.
But with all that said, why don't you use the Egypt distant terrain?