On the Garmin click on sensors, click power, and select your trainer, making sure it’s switched on and lights flashing to sync it.
It should then be connected to the Garmin unit.
I created a indoor bike setup on the Garmin, select free ride from the menu and it records when you press start button. Press end button when you finish your ride.
After your ride, save both the Zwift ride as normal and then the ride from the Garmin device.
Goon to garmin connect and you will see both rides recorded.
Openthe zwift one with the map and make a note of time, distance and elevation.
Copy the route name if required and the open the indoor trainer ride.
Click the 3 dots top right, select edit.
Change distance and time if required to match with the Zwift ride.
Continue to bottom of page and there is a button to further edit.
Click that to change elevation, calories n liquid if required.
Press save. Add photos if required.
You an then elite the Zwift ride if required as it will double your distance on garmin. 
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