just grow it out. after you change to 12/12 then about 2-3 weeks after that you should be able to see if it has white pistils(female) or little gonads(males) cut the gonad infested plants and thank the females
you dont need to switch 12/12 and then again 18/6.plants will reveal sex under 18/6 when they are mature - for most plants under 18/6 this will happen during 4th-6th week from seed,check them for pistils or balls after their 6th or 7th node