Buy or borrow a handheld GPS unit with a data transfer cable. The key features to look for are the ability to add waypoints and to add notations to those waypoints. You will also need software to transfer your tracks and waypoints to your computer. I highly recommend Topofusion, and the following steps are specific to it, although they should be similar in other programs. To layout your maps you will need a layput program, I use Adobe Indesign CS2, you might be able to use a word processor as an alternative.
Step 2: Topofusion software
Available from www.topofusion.com. Allows you to download GPS tracks and waypoints. Overlays tracks onto topo maps and satellite images pulled from the web. Gives you mileage, elevation profiles, speed, slope, time, and all kinds of good geeky stuff. Even allows you to create trail networks and run multi-use trail simulations.
Step 3: InDesign
Good layout software. Available from Adobe, part of CS2 as well. Very useful for creating a master layout and then easily adding images and text. There are other alternatives out there, use whatever you like.
Step 4: Go for a Ride, or a HIke!
First I like to clear the memory on my GPS so I don't have to do any editing later. Start your GPS at the trailhead and mark it with a waypoint, add a comment "trailhead" to the waypoint. Now go for a ride and mark each intersection or important feature with a waypoint and a comment. I use abbreviations like "TL" for turn left at the tee. Make sure you don't lose the signal or you can get jumps in the track. When you are done turn the unit off ASAP.
Step 5: Download track into computer
Use Topofusion, shown here, or your chosen program to download the track of your trail. You can set a number of things like track color and the like to what you prefer.