After a good warm-up in which the intensity slowly increases you take 5 minutes of recovery. After that just go all out for 20 minutes. Approach it as a time trial so try to distribute your energy as equal as possible. 95% of the average power and heart rate over these 20 minute should correspond with your FTP and threshold heart rate. Try to find a long preferably slight uphill stretch with not much traffic. If you do it indoors, don’t forget to disable the erg-mode. Otherwise, you will never be able to improve your FTP. Most importantly is to do this test only well-rested!
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Try the JOIN cycling app to export the workout to Zwift, Garmin, TrainingPeaks and Wahoo.