I think your example has just about everything you need in it right now. The most important part being the grid made up of the waterways I see. If you make yourself a nice grid in photoshop you can keep your graphics aligned to it well.

I make a straight horizontal and vertical grid, rotate it 45% then I transform it vertically, squashing it down until I have a perspective I am happy with.

You can do the same thing with a striaght up map, one where the features are all in outline and your view is looking straight down at it, or flat to the view. Then you can do the same thing you did with the grid, rotate it then squash it the same degree and amount. This gives you an outline of your terrain to paint in. Draw your mountains and trees in as though you were looking at them from the side and you are 90% of the way there.