Although the method that Wapping linked to does work the selection of the remaining way-points isn't done in an intelligent manner, just average distance (unless they've recently updated the algorithm). I find it serves me better by just tracing over the track with another route selecting the points I want rather than those arbitrarily selected by Basecamp.
The alternative to all this, if you prefer plotting using Google Maps, is to use ITN Converter or TYRE. Of course, these are PC only so if you are using a Mac then you'll need to find another way. The above are both capable of producing the GPX file that your Garmin needs.
Moving a GPX route to your GPS can be done by plugging you GPS into your computer via USB and moving the GPX file to Garmin/GPX/ on the internal memory. You can also create the same directory on your SD card. I'm making the assumption that the Nav5 works the same way as the Zumo and Nuvi ranges in this respect.
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