Here is something useful. A video about hot to do autofocus microadjustment.
The cereal box and ruler method allows photographers to quickly visualize where their lenses are focusing and to verify if they are out of alignment. This video demonstrates the philosophies of MFA (micro focus adjustment) and some of the principles to become familiar with if you decide to try this on your own.
If you prefer there is a neat written tutorial about how to do autofocus microadjustment by Canon itself and you can download it here.