Before you send it in, what model camera body are you shooting? Does it have micro adjustment capability? You might be able to do it yourself following any of the procedures available on the internet.
Here are a few...
http://learn.usa.canon.com/app/pdfs/...QuickGuide.pdf and this one...
AF Microadjustment Tips.
If you are technical enough to note it is back focusing, I believe you are more than capable of performing the required testing and fixing it yourself, if your camera body has the micro adjustment feature.
If you don't have a camera that has that capability, then you need to send it to Canon. The procedure is located here
Canon U.S.A. : Online Product Repair Request, Tracking or Approval.
You will need to pay for shipping and insure the lens (typically for replacement value) at UPS or FedEx (whichever you use). Make sure you place the proper paperwork (from Canon that you printed out) in the box before shipping. Canon will pay to ship it back.