Many ways to use it. Googling, bee books, Dave Cusman's site and various threads on this forum will yield different uses and methods.
I just use it as a winter extra if the hives get light - which is not often with 14 x12s full in the autumn. Nucs over-wintering will often benefit from feeding fondant.
I feed over the crownboard and under insulation. Some put it on the top bars of the frames - I tried that long ago and it made a mess to be cleared up.
Some use it as autumn feed instead of sugar syrup.
So, for most it should be an emergency feed as if fed without any due regard to the brood space, the bees may be slowed in brood development in early spring and never really catch up - always a brood cycle behind. It can cause early swarming preparations, due to lack of laying space, and then likely swarming is difficult to control.
My bee inspector assured me that roll out icing IS suitable to use as fondant, I really quizzed her about it and she was adamant that it was fine to use and didn't have any nasties in it detrimental to bees. And one pack would be enough for my hive over winter.