I'm not sure technical but here's what I can gleem of a few ideas.
You could maybe try to get them to save their documents in a folder on the desktop and use Known Folder Move (right click onto OneDrive app icon in icon tray > Settings > Auto save > Update folders and select Desktop which will then sync their Desktop with OneDrive.
You could also try having them save their documents to the (OneDrive) Documents folder which can sync with OneDrive. Or maybe even just shortcut a link to the desktop where users can open it and drop their files into OneDrive.
Hope that gives a few ideas? Not sure doing it from C drive is the way to go although it's may be possible to do!