Growing onion sets

I was reading about onion sets here this morning. I grew onions from sets in London with good success. But nobody seems to have heard of the practice here in small town.

We don't (or not usually - we are having a warm Autumn this year) have a particularly long growing season here and I had little success growing onions from seed this season just gone.

I would like to learn how to grow the sets and see if I could create my own kick start conditions. Any tips most gratefully received.