Turbo or road
Warm up well then go for the test. Ideally on a turbo for maximum repeatability, if not, a road that's either flat or a slight incline, free from traffic lights or obstructions.
You should ideally be evenly paced throughout the 20 minutes yet be completely spent at the end.
Once you have completed the test, update your zones in the settings.
Z2 Power, try and aim for at least 60% of workout time and keep it well controlled on these sessions between 55-75% of FTP