I've started playing roleplaying games with my two children and to this end I've started drawing fantasy maps. I've found the following guides by Jonathan Roberts very helpful.
Here is a general guide on drawing a map in Photoshop. The author provides a series of steps that will help to create a fantasy map of all scales.
In this article the author expands on a simple design technique for creating world maps prior to illustration.
As a bonus, here's a Wired slideshow of the sketches Tolkien used to develop his stories.