As I wrote before, I used the method from John Seymour's "The New Self-Sufficient Gardener." Here are photos to go along with the steps.
Start with dried onions. The tops need to be brown and mostly crispy.
Next you need about three feet of string I recommend something strong like twine. I used a light string for my first set and it broke the next day. Twine worked better. Tie a knot with the two ends and hang from a hook. I recommend doing it outside, as you end up with many onion skins and leaves all over by the time you are finished.
At the bottom of the string at the loop, take an onion with a long tail and tie it to the string. This is the only onion you'll have to tie.
Take the next onion and set it on top, putting the greens between the two strings.
Wrap the onion leaves around the string on the right, bringing it to the front. Then put them behind the string on the left.
Do this a few times to secure the onion.
Add another onion, and weave the leaves in and out between the strings. I found that if I continued to weave the leaves from all the previous onions in, that the weave got to too big and the onions were to spread apart.
Instead, it was better to weave the tops in some and then let them stick out at the sides, leaving room for the tops of the next onions. Make sure to add onions starting from all sides of the string.
Stop when you are getting close to the top of your string or you run out of onions.
Tie a knot with the twine above the last onion. Trim off the ends of the onion greens that are sticking out, then hang in a cool place to store.