No need to write a tutorial, it's quite straightforward. Best is if you have a detailed map where all the different elements (grass, arable land, river, roads, woods...) have a different color. Import in PS, select the ground cover (say woods) you want, make another layer, fill with white, inverse selection, fill with black, save this layer as 'wood-mask.tif' (make it gray-scale to decrease file size). You can even decrease the size of some masks if they are not to be very precise. Etc. You can always color/adjust by hand. Or paint the whole mask series by hand, which I often do.
It's easier to see if you import the total map first (image map shader) and attach this to the color input of a surface shader. Temporarily. Then import mask in an image shader (Y plan), set the size to the size of the area you want to show. Attach as blender to a next surface layer, and color/displace that layer or use as a mask for the trees. You'll manage from here on, I guess.
Things like roads need to be sharp, so you might want to enlarge (200% or more) the initial map and paint very sharp white lines where the roads are.
Hope this helps.
---Dune