In my garden this year, I went big. Go big or stay home right? I planted 5 rows of onions, several varieties, and I dried them out and hung them in my pantry to store for winter use.
Drying onions is very simple. When the green stems start to die off, or when the onion has outgrown the hole you put it in, pull the onions and let them dry for about two days out in the sun so that the skins can harden up. Air needs to reach the entire onion, so if you don't have something to put them on that has good ventilation, you might want to rotate your onions every other day.
The onions are finished drying, or "curing" once the stems have all turned brown and are completely dried up. Additionally, the skin on the onions should have a withered look around the stem. Once the onions were to this point, I clipped the stems/tops off the onions leaving about 1/4" - 1/2" from the bulb. If you don't leave at least that much space, the neck of the onions won't dry out and could possibly rot when you store it.
The next thing you will want to do is have several pair of pantyhose, yes pantyhose on hand so that you can store your onion in them. To store your onions in pantyhose, simply cut off legs of each hose, and drop an onion in one of the legs. Make a not at the top of where the onion is and repeat until the leg of the pantyhose is full. This is a very cheap way to store onions and still allow them to breathe.
After you have the onions in the pantyhose, you are ready to store them. You should choose a dark, cool area that stays between 40-50 degrees year round. For some people, this might be your basement or a root cellar. We simply shut off the heat vent in one of our rooms and block out the light from the window through the winter time. If the area that you store your onions in becomes to hot, the onions will begin to sprout, if it's too cold the onion will begin to rot.
When the times comes that you need to use them, simply cut them off the pantyhose!