I look in weekly. When I spot queen cells I move the hive three feet away, put the super/s aside and put a new brood box filled with foundation in its place. Find the queen and put the frame she is on into the middle of the new box, making sure there are no queen cells with her. If there are queen cells on that frame I just put her on one that has none but has preferably sealed brood. I usually add another frame of capped brood but you can get away with just the one. Replace the supers on the new box and close up. The bees will have drawn the frames and the old queen will have that box laid up in no time
Returning to the old hive, I look through the queen cells and choose a nice open one with a fat grub floating in lots of royal jelly, mark the frame and destroy the rest.
Make sure there is food in the brood box as this box will have few foragers for a number of days. Sometimes I put one of the supers back on this hive. If you are dealing with just one flow super you might have to feed this box for a week.
I go back in 5/6 days to check both boxes because they may make more queen cells.
What I have done is separated the queen from the brood. The queen right box has all the foragers and will continue to bring the honey in. The old box will make themselves a new queen but you still run the risk of them throwing a cast swarm if they are strong which is why you have to return to knock down any more queen cells they make (they invariably do.)
Sometimes your queen right box still has a swarming urge so they make another few queen cells and still swarm so you have to knock those down. This is the usual reason for this system failing but if you are on top of it it is a really good method of stopping your colony swarming and keeping your honey.
Once your new queen is up and running you can either kill your old one and unite through newspaper or pop her in a nuc box and keep her as a spare.