Not to speak for Felwynn of course, but that might be difficult to do entirely, given the artistic, painterly nature of the style. But also given that nature, you'd do well to find a book or browse the internet for tips and tutorials on painting in general and more specifically for landscape features you have in mind. YouTube is invaluable that way so you can watch someone else do it, or, if you want a really valuable book on painting, take a look at James Gurney's Color and Light.
It doesn't really matter whether the tips are for photoshop or natural media if you plan to use photoshop or similar raster image creating software. The concept is basic: You're trying to create a 3-Dimensional area on a 2D surface. Just think of this type of map as a landscape painting viewed from a higher angle and somewhat condensed, and then paint accordingly.
You could start with a dark mid-tone and work toward the highlights as Felwynn is doing, for the most part, or start light and go dark like a watercolor... it's really just working preference. Strong lights with shadows pop forward, cool colors recede. Try to keep your color warmth pretty even throughout (if your light source is warm, your shadows are cool, plus or minus local color of objects, bounce light and reflective colors). It might help to start if you find a painting or landscape picture you like the color scheme of and pick your colors selectively from that for your brushstrokes. It also helps if you start with a loose sketch, whether a line sketch or a value sketch (paint it roughly in greyscale first then modify the layer with a hue adjustment to something close to the base or most abundant color in your intended piece so it won't stand out so much if you miss a few places) and then paint on top of it... kinda like a coloring book or paint-by-number
That was probably less than helpful, but maybe not.