As burns375 mentioned, it is common practice to feed NUCs for awhile to help stimulate them building comb, especially if you are using new boxes and/or frames. The presence of the feed or nectar stimulates them to draw comb quickly. As I mentioned, they don't always wait until the comb is drawn to start filling with nectar. I have seen foundation with just barely a hint of comb, already loaded with nectar to the brim of the new partially drawn comb. They fill and draw simultaneously. So Don't wait and don't fret. Either foundation or empty frames will do for the bees. They're not picky!
As to feeding methods, there are many. But it doesn't take fancy feeders to do the trick. It has been my practice to simply take a mason jar and fill with sugar water/honey. I will often add a few spoons of honey to give it flavor. Poke 5 or seven holes in a small pattern about 1/8 inch apart,in the lid with a push pin, make it a small enough pattern to fit between two frames or be available to the cluster through the hole in the inner cover. Then secure the lid tight so there can be no air allowed in, except through these small holes. Then simply invert the bottle over the hole in the inner cover, or place directly on top of frames. During cold months, I will place an empty box on top of the inner cover with the lid on top, to keep the jar out of the weather. The heat rising from the cluster helps keep the contents from freezing, and warm enough to be consumed without chilling the bees. Simply lift the lid occasionally to check the fluid level and replace as necessary until the nectar flows. They will automatically stop drawing from the bottle when the nectar is flowing, or they go dormant. I prefer this method because it is easier to judge the amount of feed available, and you can lift the lid and replace or take the bottle, without opening the entire hive to the elements, to fill or check a frame feeder. Additionally, the bottle method has the lid right at the top of the frames where the bees are, rather than having the bees have to move up a ladder and into a top feeder, or over and down into a frame feeder. Movement during the coldest months is very difficult if even possible. I have succeeded in keeping a small cluster the size of a baseball, alive until mid March using this feeding method, while supplementing their heat with an in-hive warmer. The small cluster never moved laterally, and stayed right on the bottle the whole time. The only reason I lost them, was because of an extended power failure during sub freezing temps.
For the sake of critics, my example was extremely a-typical, and nowhere near the size of NUC that most any beekeeper would invest time and energy in. I was, however, running an experiment, and trying to save a late season swarm that appeared to be a desirable mix of italian and Russian or Carniolan. My little experiment proved to me that it was possible to assist a small colony in surviving a cold Utah winter. Even though I ultimately lost them, it was only after succeeding at keeping them alive through the harshest part of the winter. My little queen was in the small cluster to the bitter end. So I missed by only that much... The point of this comment is referring to the bottle method working through to the conclusion of that case, and my little cluster would not have been capable of moving sideways and down into a frame feeder, or even up and then down into a top feeder. The bottle seems to allow for the smallest amount of movement in a survival condition, in my opinion and observation.:thumbsup: