How do I make a modern(ish) map with photoshop?

[Fragilicious]

Hi Forum,

The campaign setting I created for my D&D games has evolved over the 12 years or so I have been using it, and now the main continent is a lot like 1940s Europe. However I'm still using the map I made for the medieval era, a map I used right through the renaissance, and into the more modern games. I want this to change, so my question is as follows.

How do I turn this:


Into something like this fine, upstanding member did:
http://www.cartographersguild.com/fi...-antillia.html

Now I know that Lingon used watercolours and whatnot but I'm looking to keep this in photoshop. I'm going to start tackling it myself and experiment with a few methods (that I'll eventually figure out) and I'll even put together a tutorial to upload if I manage it. I'm also moderately confident in my 'analogue' arty stuff skills so I'll turn to that too if it needs to leave photoshop (though I'll need to buy supplies). Either way what I am getting at is; has anyone done this before me and are able to enlighten me, or does anyone know how to achieve it and again can they explain it to me.

Duly appreciated

Frag