Garmin connect has a course creator tool. It's easy to crest your course with mouse clicks on a map and then just export the course to your device. Then you will get all the turn by turn info. Higher end devices with map pages will show it on a map.
I know of ridewithgps, mapmyride, etc, where you have to manually create point-to-point mouse clicks to create a map, but I'm looking for something a little more automated or systematic way. The route I have is 60 miles long, and somewhat technical, I can't imagine clicking on a map all that activity!
I have an edge 500 and have used this in the past to pull a gps file convert to a course and push it to the 500. Worked pretty well for an 80 mile ride but I kinda knew where I was going and the 500 is not really meant to do this very well.
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Oooohhhh....good question....I'll be lurking.
I've created cue sheets for my IM camps using RidewithGPS.com:
I imagine they have a process whereby you upload a GPS file, it puts on a map and generates a CS from that? Or you can just us my process above.
http://tcxconverter.com/TCX_Converter/TCX_Converter_ENG.html
How about this for the 510?
http://velogps.com/garmin-edge-510-course-navigation/