Garmin's tutorial is here:
http://www.garmin.com/garmin/cms/site/us/onthetrail/custommapsAlso see the following software which makes it easier (although I haven't used either):
http://topofusion.com/garmin-custom-maps.phphttp://moagu.com/?page_id=155As Maps4GPS has said, this only works on the Oregon, Colorado and Dakota. There really isn't any practical way to do this with other Garmin models.