Hi there! I recommend you follow The Winter Survival Plan (For ducks!) [Printing this off might help too]:
1# Have their housing well insulated. Getting thick foam is a popular option which works great, but you can usually put basic material, bin/trash/garbage bags around the coop so that no sharp cold winds enter inside. Make sure you have their vent uncovered however, so that they have fresh air circulating into the coop. If you have the time, you may wish to consider putting a heat lamp inside the coop. Infra-red lamps are good for really cold winters and your ducks will definitely appreciate it too! If not, put a simple basic light bulb in there, the excess electricity will generate some heat for the ducks.
2# Give them a good supply of food. Try to give them foods that contain small amounts fat and carbs such as bread. It will help them build up a strong body. Also, give them a good amount of protein, this builds their feathers up and gives them a warmer coating. Sometimes a warm snack is beneficial too!
3# Water. Your water must not freeze! Your ducks need fresh water in order to survive. It seems simple but is not always that easy. Try put their water with them not outside in the cold. Check it regularly and if has frozen, pour hot water over it to melt it. If you cannot check on the water because you might be busy, fill your waterer with warm (not hot) water, this will slow down the freezing rate, and your ducks will like it.
4# Baths. If your ducks have a bath/pond/river then try to make sure it does not freeze. It is not crucial but ducks do like their baths! Having a frozen bath will also make things harder for you to manage your ducks. Installing a heater is effective but costly. Instead, you could cover them up with plastic sheets or something similar.
5# Be careful with snow! Snow won't harm ducks at all but can make them wet and cold, which will in turn make them ill. If it has snowed heavily, consider letting your ducks stay inside for the day. However, don't feel that you should keep your ducks in 24/7 for the whole winter. If you have got the time and it has stopped snowing, clear out the snow outside the duck's coop, they can then go outside and enjoy the space without getting wet and very cold.
6# Be one step ahead of mother nature! Check the weather forecast often so you aware when the snow or ice arrives. If the cold weather is a week or so away, collect more feed from your suppliers or the pet store to keep you going in case you cannot travel. Try to insulate the coop at this stage too! If the snow is set to arrive within the next few hours, insulate the coop quickly, check the waterers and keep them away from the snow, make sure you have enough food to keep you going, or visit the pet store for some food, even if it is poor quality, buy it! It will help.
7# Give the ducks some fun! Winter can be quite restricting for ducks, they cannot go out and feel uncomfortable in the cold. Giving them something fun usually helps. A popular choice is to give your ducks some warm treats. You could place an "unusual" object in their coop that they will play around with. Another choice is to put a heap of straw inside, which they will happily play with and make nests out of. Creating a comfy nest won't normally make your ducks lay in the winter but it will give them something to do and they can rest in it when it is cold.