Turbo (trainer) 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.
*If on an indoor trainer* If you can record 'distance' on your trainer then do so so you can compare in a similar test at the end. If it is a wheel based turbo make a note of tyre pressure so it can be the same. If you are on a gym bike, try and use the same bike for the final test.
*If on the road* Make a note of weather conditions, your weight and traffic levels to assist comparison at the end of the program.
These tests are where power meters really help as they take out all the variables.
This test is based on rate of perceived exertion, 1 being easy, 10 being a flat out sprint. If you have a previous threshold heart rate then use that as a baseline. If not, then you should be permanently in a state of thinking "I'm not quite sure I can keep this up for the remaining time"...
Once you have completed the test, update your zones in the settings.