You don't. Nature does it for you. That's why seedbanks give estimates for flowering indoors (eg. 55 days) or outdoors (mid September and such). Peace.
The length of the days change throughout the year, longest during the summer and shortest in the winter. The lack of light ( 12 hour photoperiod ) is what causes them to flower.
So, you can either cover them for 12 hours a day until the day of harvest, or wait until nature takes over.