Once you get the hang of actually setting it up in blender, it's surprisingly simple: pick a few colours, save them to a palette, paint away. For tiling environment art (ie the entire pack in this case), I prefer to just add a quad, set up the material/texture for painting, and use an Array modifier to duplicate it out. Enable welding and tadah: free tiling!

As for specific blender painting techniques, the most important thing is to mess with the falloff curves, because the default curve is kind of awful. Otherwise I recommend texture paint tutorials on youtube: this is an excellent quickstarter. Block your shapes out with large brushes and go smaller as you're adding detail. That's more or less it.

With low poly meshes/textures (f.ex the cliffs), try to paint in straight lines and blocky shapes that you can follow when modelling them out properly instead of just painting on quads. Knife tool and the vertex slide tool are your friends here. Once you have a cliff piece / treeline etc you can also abuse the array/curve modifiers to quickly hash out the landscapes you want.

Thank you kindly! Never actually played that, but a Blizzard comparison is hilariously apropos