First you do good warm-up in which the intensity slowly increases and some very short sprints. After that just go all out for 20 minutes. Approach it as a time trial so try to distribute your energy as evenly as possible. Your FTP and threshold HR are calculated at 95% of the average power and HR over these 20 minutes. Ideally, do this on a slight uphill stretch of road that is quiet and long enough. If you do it indoors, don't forget to disable the ERG-mode, otherwise, you will never be able to improve your FTP. Most important advice: do this test only when you are well-rested!