Easiest way I know of is to remove one of the top bar frames and then set the nuc atop the top bar hive so the bees have enter and exit the hive through the TBH. They will expand into TBH and after a few weeks you can remove the nuc, or if it still has brood in it let it queen itself. Once there is adequate comb in the TBH the queen should go down on her own.
Genious!!! I have been wondering for some time about this myself, with no good answer. Any problem with bees messing up the comb?
What about making some special top bars with notches in them for this purpose? ie:
(all the llllllllllll's imply solid wood, where the spaces are a gap.)
Put 5 of these top bars next to eachother.
Now take the bottom off of the nuc, or build a bottomless box to hold the frames.
put the nuc over the holes. when the bees expand the broodnest into the tbh, check for the queen in the nuc.
once you are sure the queen is in the tbh, put a queen excluder between the nuc and the tbh.
wait for the brood to finish hatching in the nuc, and then remove it.
Maybe we can keep the ball rolling here and someone else can make this even better!