Creating an actual map from top-perspective. How?

[Gamerprinter]

If you're wanting to make an isometric map then I would take a look at these two wikipedia entries:
Isometric projection - Wikipedia, the free encyclopedia
Axonometric projection - Wikipedia, the free encyclopedia

In particular this image should illustrate the differences between different types of projection. Once you understand the basics you can do it one of two ways. You can freehand your design, keeping the projection type in mind (this seems to be how most artists here do it). Or you can put down a grid and use that as a guide. I would say the grid is more useful with man-made structures than organic ones.

I cheat. I use a 3D application to create and place the objects that will be used in the projection, then I set the camera to isometric mode which creates the proper angle and the non-perspective view. Of course I then usually print the image in b/w and trace it with a pen on tracing paper to rescan and create a quality hand-drawn isometric map perfectly.

Here's a link to my March 2010 CG Map Challenge Entry thread which I create one of these...