Create a course with cues where you want them - RideWithGPS is my favourite for road and trail.
Export as TCX - A subscription with RWGPS will let you offset the cues if that is what you want.
Convert the TCX to FIT - Garmin Forum member mcbadger's CourseTCX2FIT for pace then JaVaWa RTWtool for compatibility.
If a 910XT (ANT+)
Copy the FIT to the hidden folder /Users/YouHere/Library/Application Support/Garmin/Devices/910IDHere/NewFiles
Sync with Garmin Express
The transfer will be complete when the FIT has gone from the NewFiles folder, a sync is needed for each file if more than one, additional sync's may be needed to get past error messages.
Copy to the device NewFiles folder, eject and power up.