The easiest way is with a foam cover that holds onto the spigot with a cord.
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If it is unusually cold, old timers would leave a small drip of water running, since moving water does not freeze. I would only do this in an extreme situation.
If you don’t cover the spigot, and it does freeze, you will have to have it repaired or replaced. this will cost a lot more than a cover. It is also a good idea to drain your hoses and bring them inside, or in a shed or garage, where they are shielded from the elements.