I have a Garmin Edge 605.
Depending on the site/source of the map:
* If I have a KMZ File (Google Maps) -- I use http://www.gpsies.com/
and use it to convert it to a Garmin CRS file,
* If I'm using Map My Ride -- "Tools" - Save To Garmin CRS,
Once I have the CRS File, I'll load it into the Garmin Training Center Software and then send it to my device.
I haven't figured out how to load a turn-by-turn onto it though which would be handy. Some times looking at the map, I'm not 100% clear if I should be going straight/veer right/etc, and having that kind of prompt would be handy.