switching to flower need help!

so im about to start the transition to flower, my plants are all about 2 feet and are sizeable to my liking, my question is, do i have to change the lights gradually or should it just be a straight transition from 18/6 to 12/12? and when do i change the nutes to flower, before the lights, or after the lights or at the same time as the light transition.

bump need advise please

Just switch to 12/12 no need to gradually change the photo period

And the nutes start the transition now as soon as you switch to 12/12 and then around 2 weeks into flowering start full flower nutes

i just switched to flowering and i gave my babies 3 continuous dark cycles (LIGHT-DARK-DARK-DARK-LIGHT)...
the results were pretty cool, all my plants grew about 4 inches during those 36 hours of darkness and grew many branches!!!! i couldn't believe they grew that much in 1.5 days (they didn't grow that fast even in veg!!!!!), you can start feeding the flowering nutes after the plants know they are supposed to flower (afar the 36 hour dark period).

Just go 12/12, as for nutes that is up to you. I like to keep mine a little greener till the end so I keep up with the veg nutes until I see pistils on multiple nodes. You did not mention if you sexed them yet, to be cost effective I would do that first as it could save on nutes and still allow you time to take a clone if you wanted to.