Originally Posted by jaxgtr

Spacers are about $3 if needed. I lost mine once when I was cleaning my cassette and forgot to take it off the hub and then moved the wheel, it came off and is garage somewhere. I went to my LBS and they just gave me one. Shimano has all the cassette order diagrams on their site, here is the link to the 105 cassette which is probably on your bike. Each sprocket has the tooth count on it. It's really really simple as there is no way to screw this up unless you can't read and purposely put the sprockets in the wrong order.