I think the most easiest way to upgrade Fedora 11 to Fedora 12 is program called preupgrade, which download needed packages from the server and then just reboot to installer and after install boot the new system. Of course it's important backup your important files before upgrading.

Commands and screenshots of whole upgrading process

First start with updating all Fedora 11 packages with following command as root:

yum update

Install preupgrade as root:

yum install preupgrade

Start preupgrade as root:

preupgrade