If you have not already done so be sure to take in your onion crop now, as they will rot if left in the damp soil. When lifting your onions they should be dug out gently with a fork, as pulling them can bruise the flesh, resulting in rot developing in storage.
Having lifted the onions leave them to dry in an airy place under cover. Any that are misshapen or look like they might not keep, lay to one side and use these first. The remainder can then be tied up and stored in a dry shed for use over the winter.