You can download routes from plotaroute.com as GPS files (in various formats) - select "Download" from the menu above the map when viewing the route. How you transfer this to your device will depends on the features of the device you use - you would need to check the instructions for the device. However, most SatNavs don't support importing pre-mapped routes as they are designed to calculate routes themselves. You might be better looking at a device like a Garmin Edge if you want to import a route for navigation.
I've planned a route on here that I am planning to follow through Europe on motorcycle, it is about 2100 miles long. I'm going to be buying a TomTom motorcycle sat nav, I was wondering how I download the route on here onto it so I can follow it throughout the day?