The fantasy cartographer Jonathan Roberts has a blog post, reprinted at Tor.com, that shows some of the steps involved in creating the map for Sabaa Tahir’s novel An Ember in the Ashes.
I sometimes get asked how to do a fantasy map. I’m the wrong person to ask, because I’m basically a fantasy map critic, not a working illustrator. What the people asking me this question want is an instruction manual for the standard fantasy map, and for that, Roberts is their man, because he’s an actual illustrator. He does operate within the dominant fantasy map paradigm I often critique (though with a good deal more colour and texture than the standard line drawing), but he does it very well, and more importantly shares his methods. Roberts’s blog is full of interesting material on how he goes about creating fantasy maps: see for example this tutorial.