In reply to GrahamD:
Garmin units accept two types of file: GPX & TCX. The latter is a superset of the first and can contain "turn by turn" instructions. It's possible to create a TCX file from a GPX file.
Create the GPX file in whatever app or web page you prefer.
Export the GPX as a file
Go to bikeroutetoaster.com
Import the GPX file and convert to TCX, fill in the details of how far ahead you want the prompts to appear: "Turn left in 50 metres" type of thing.
Save the TCX file
Send the file to whoever. In order for them to load it in to a Garmin:
I'll assume that the user has saved the file somewhere in a folder named like bike-routes and that their unit appears as a USB drive on their computer.
Plug in the Garmin.
Drag the file from "bike-routes" to the "new files" folder on the Garmin.
Unplug the Garmin
Reboot the Garmin.
The route should now appear within the list of "Courses" on the Garmin.
Edit: there's one of the online apps that creates GPX files that Garmin units don't like but I can't remember which one it is.
Post edited at 11:23