To flower, you need 12 hours of complete uninterrupted darkness every night. It's very simple, 12 hours of light, 12 hours of dark, repeat until harvest. Once you start the flowering light cycle (12/12), do not change it.

If you want to have some plants vegging and some plants flowering at the same time, you need two separate grow areas on their own lighting schedules (one with 18/6, the other with 12/12).