I learnt how to do this last year from this site:
Give it a try - it's the best way to store them
Sorting out the sizes - Large, Medium & Small
The first string in progress
After trimming off the roots and the ends the first string is ready
Three Strings later (the one to the far right are the over wintering onions) and I had to call it a day as there are about another 5 lots to go!