Question: How to Create a Better Trail Map?

Brod Berser writes:

I have the [National Geographic] Topo! California regional software and a Garmin III+ GPS. I would like to make a trail map for Torrey Pines State Reserve here in San Diego. There is a hand-drawn map but it is not much use. I need information on a reasonably accurate method to follow, or a map source that could provide a better map. [Link added]

Basically, Brod is trying to avoid reinventing the wheel. If you were to go about making a good trail map, how would you do it?