There are several ways to do that; either make your own (Photoshop) 16-bit greyscale heightmaps, imported as image map shader, and used for displacement or mapping. Or kneed a mountain in another software and make a vector displacement map, import and vector displace it. Or use the painted shader (or warped simple shapes) to shape certain parts in TG itself (warped simple shapes are good for rivers, e.g.). And after adding a compute terrain, you can laterally displace the terrain by internal power fractal masks, or painted masks, or imported greyscale maps.
But TG doesn't work like ZBrush.
Or have a little patience (can't say more).