Learning to paint high res terrain textures

Hi, all.

I’m trying to learn producing high resolution multi-textured terrain assets for games. As I understand, to make it really nice looking, like in the reference I’ll post below, they need to be hand painted.

This is my reference image, I want to create something similar to this. Although I need mostly plain terrain, terrain like mountains and hills will be actual 3D models.

Would really appreciate advice of paint artist, or links to terrain painting tutorials, for some reason didn’t manage to find anything useful on google.

Thanks in advance.