1) Avoid syrup at this time of year. They need to be warm and active to dry it down.
2) Fondant or dry granulated sugar works well in winter. Personally I buy blocks of fondant that I wrap in clingflim and place directly on the frames with a slit cut into the plastic.
To give them dry sugar place a sheet of
newspaper across the top of the frames - leave a small part uncovered for bee access. Damp it with a light
spray of
water. Pour the granulated sugar on top in a heap. Lightly
spray the top of the sugar to make it moist. The damp will hold it all together in a clump.
3) Be sure that they actually need it. Don't open them to inspect, but you can heft the boxes to feel how heavy they are.