Feminized plant. What do I do?

You should be on an 18/6 schedule, so you're already good there. When you want to go to flower, you flip to a 12/12 schedule. Once you flip, you'll be in preflower. The plant will show flowers in a week or so, then really start stretching for the next two weeks. Generally, it grows 2-3 times the size it was when you first flipped. So if you want a 20"-30" plant, flip around 10"-12". If I were you, I would top now if the plant is in a good position for it. Then give it 5-7 days to recover, then flip.