Before introducing your new queen, it's essential that you identify whether the old queen is still present or not - for if she is then you're almost certain to have a queen-fight on your hands - and 'Sod's Law' (aka Murphy's Law) says it will be your new queen who will be the loser ...
A pretty-much guaranteed way of finding your queen is to place two bee boxes one above the other, with a Queen-Excluder between them, and with a base under the stack.
Then shake the bees off a couple of combs - open brood combs (without ANY bees) are best, if you can pinch a couple for this exercise - and place them in the bottom box. Place the QX on top, and another box on top of that.
Then - systematically - shake ALL the bees from the combs of the hive in question into the top box. Place a towel or some other cover on top if they're a bit slow to descend into the lower box. Eventually you'll be left with a handful of bees on top of the QX (there's always a few 'difficult' ones), together with any drones who may have moved-in as uninvited guests. At this point you should be able to tell whether there's a queen present or not.
If there is, then catch her and make-up a very small nuc, if you want. She may just be a very slow layer, and a spare queen is always handy.
But if you've established that the queen is missing, then re-constitute the nuc you've just dis-assembled, and return it to it's stand. It may be a good idea to include those open brood combs in this nuc, providing the nucs you robbed them from have enough brood for now.
Then, you'll need to introduce the new queen. If the old queen WAS present, then leave the nuc queenless for a couple of hours. If she WASN'T, then just carry on - and - hang the mailing cage inside the nuc. (I prefer to place the cage above an inner cover open feeder hole, with an inverted small plastic food container over - within another nuc box or feeder shell - to make inspections more convenient).
Then - and this is the really clever bit (not my idea, BTW, I got this tip from a Dutch guy via the Internet) - each day, very gently wipe a tooth-pick, skewer, or similar, over the top of the mailing cage, where the bees will have clustered like hedgehog spines. If there is ANY resistance as you wipe the skewer across, then the bees are still gripping the cage and thus are still 'anti-queen' - so replace the cage and try again the next day. As soon as you are able to wipe the skewer across the top of the cage with the bees obligingly lifting their feet to let it pass under them, that is a sign that they've finally accepted the queen's presence.
Although the source I got this from said to release the queen at this time, I still prefer to wait another day, as belt and braces, then simply open the mailing cage and let her out. To my way of thinking, that's a far preferable method that relying on candy-eating timed-release and just hoping for the best.
Good luck
LJ