First grow your onions...easier said than done in my experience. I tried growing onions from seed for the first time this year. They are an Italian torpedo shaped one. They didn't grow very big but there were just enough to make a string. Globe shaped ones would be a lot easier to make a decent string.
Anyhow here's how to string your onions.
Cut a 4 ft length for a two foot string.
Fold it in half and tie the closed
loop end around the neck of your first onion.
Tie the two loose ends together to form a continuous loop.
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.
Now take your next onion and in the same way wrap it around the right and then the left strings with the bulb dangling alongside its neighbours.
Carry on with successive onions.
Once you have run out of onions or feel the sring has got too heavy, take the string off the peg and tie a knot shortening the string loop to the top of the onion leaf plait.
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.