I usually buy the feathers that come in pairs so you get a single feather from the left wing and one from the right. A good quality pair or set will be exactlly opposite each other in size and shape which makes taking the quill section from each one to make a wing set much easier. Here's the quickest thing I could find to explain. I'll do some more looking.
http://www.flyfishersrepublic.com/ty...her-slip-wing/
http://www.eugenecarsey.com/flytying...rs/quality.htm