It's really easy and will save you a few bucks since you won't need to buy them every time you go to the store.
Here is the before picture. We had used up the onions in salads and other dishes. Before I learned this little trick, I would have thrown the roots away and bought more during my next visit to the grocery store.
I kept the rubber band around the bottom of the bunch and then filled a glass with about 2 inches of water. Every 2 or 3 days you should dump out the old water and refill it with new water. The onions need to be in a place of moderate sunshine.
Onions don't grow at the rate of other plants. These can easily grow 1/2 an inch every day.
4 days later, this is what I had:
I'd say that this is enough onions to last me a week or so. I've already cut off those onions and have new ones growing.
Here are some useful references: