First, make sure your pot is big enough for the calla lily bulb to multiply, which it will definitely do. That's the biggest mistake most people make when potting them. To winter it, just quit watering after the first frost hits the leaves and they will wither and die back. A few frosts are ok but get them in before the freeze. You can keep it in the pot (don't water it) over winter in your basement or anyplace where it's cool and dry, and repot in spring. Or you can dig it up after the leaves die and dry up and keep the bulbs in a cool, dry place, separated from each other such as in a box of wood shavings.