I have been using RidewithGPS for years now to map my routes and download the .gps files for transfer to my Garmin. Once you make your route simply download the file and copy it to the gpx folder of your garmin device via USB cable (this is for owners of models that accept user defined routes - most Garmins do not) Now import the route using the Garmin My Data menu.
For complex routes where the .gpx route file this site produces is not detailed enough there is an option to download the route as a track file. You need to then reduce the number of way points from a the breadcrumb type 1000's typical in a track file to a more manageable figure like 100 and convert it to a .gpx route file.
GPS Babel is free software that will do this for you. Simple select your track file and convert it to a route file (the extension remains .gpx) and when converting select limit waypoints to 100. This will create a file with a very detailed set of points for the Garmin to follow so it wont get tempted to short cut you via a highway when you want to ride the mountains. Now transfer this file to your GPS and import it as a user defined route via the My Data menu or the Zumo models will import it automatically for you.
Sorry but I have elected to delete the rest of my original post as it was entirely out of date and thus misleading.