Just keep the concrete from freezing for 7 days. It reaches 3/4 of it's strength in that time. After that the frost won't harm it. Concrete makes it's own heat during curing. Covering it with a poly sheet is all you need to do as long as the ground isn't frozen to start with. Add a little straw if you really want to make sure. Sometimes the poly blows away in the wind so throw a few shovels of dirt on top.
