Here's a video tutorial I put together that plays in Windows Media player to get you started making basic maps by tracing map features in Google Earth and then making and installing the map into your Garmin GPS.  Once you learn the basics you can add more detail to your maps.

Download Video Tutorial Here..read the READ_ME_FIRST.txt file

http://www.mediafire.com/?e1gx2gd0k7vDownload

You'll need the following software packages:

Google Earth...trace features you want to appear on your Garmin map

http://earth.google.com/

GPSBabel...used to convert kml to gpx 
http://www.gpsbabel.org/

GPSMapEdit...program used to make your map
http://www.geopainting.com/en/

cGPSMapper...used by GPSMapEdit and MapSetToolKit
http://www.cgpsmapper.com/

MapSetToolKit...program used to install your map into Mapsource
http://cypherman1.googlepages.com/home

Go from Google Earth map to Garmin Custom Map

Custom Map shown installed in Mapsource

Screenshot of Custom Map from a Garmin 276c