Start with poly plane. Use default settings, scale the plane.

Then go to sculpt geometry tool.

Deform the plane to get the form of terrain.

Smooth the plane. I did twice (level 2 smoothing)

Add detail to mesh

Then go to UV selection mode.

Grab all UV's and go to UV texture editor. Then do one Relax to UV's

Now UV's match the mesh better.

Assign lambert material to surface. Then go to 3D paint tool. And click assign/edit texture. Make 1024 texture map.

Use flood paint button to paint whole surface in white.

Then add black parts, on places where will be mountain top.

Then assign another lambert shader to surface and paint areas where grass will be.