Fold it in half and tie the closed loop end around the neck of your first onion.
Hang the loop from a peg with the onion weighing it down from the bottom.
Take your second onion bulb and hold it next to the first.
Feed the onion top through the loop from front to back and then wrap the onion top all the way around the string to the left and then around the string to the right in a complete figure of eight maintaining downward pressure as you go. Leave the excess dangling. It can be trimmed later once well dried if you are worried about appearance.
If I can do this with these torpedo onions anyone can do it with ordinary ones!
I learned this technique from "Vegmonkey and Mrs" a blog that stopped over a year ago - I'm keeping the information flowing around the net.