Here an illustration of making Terragen picture step by step.. from a simple heightfield to a complete procedural shaped terrain and texturing
This scene is available in my store ready to play with it and add new iterations !
Starting by create global procedural shapes
Adding details without denaturate the landscape
Adding colors by altitude and/or by slope
Adding some vegetation (here some XFROG plants used) and adjust colors to better match. Find an interesting point of view.