Open a browser on your phone or PC and go to the website cycle.travel. Create a login and password then log on and create a route. When finished, select “Save Route” to give it a name then select “Download for GPS” and “Send to Garmin Connect”. Open Garmin Connect app on your phone and you will see the route under “More”, “Training & Planning”, “Courses”. Hit the course, it will open and you’ll see at the top right a mobile phone with an arrow, if you hit that the course will be sent to your Garmin. Note that it has been converted to a proper .fit route not a track.
While you can use cycle.travel on your phone, it is better to use your computer because you can click “Round Trip” to create a loop returning you to the start. Note you can also select the little mountain icon to show elevation profile. Also, after creating a route you might want to select “Set to Paved Only”.
Cycle.travel works in most countries and has never failed to give me the most scenic and safest route.