The process depends on the exact versions you are working with, because some upgrades have specific requirements. The platform is also important.
This article gives a very top level view of the possible approaches. It was written for 11gR1, but the overall approach is similar in 11gR2.http://www.oracle-base.com/articles/11g ... to-11g.php
Prior to 188.8.131.52, patches were not complete installations, so you would need to upgrade to the base release then patch to the latest patchset. From 184.108.40.206 onwards, patches are full installations, so you can upgrade to the latest patchset directly.
For detailed instructions you need to read the upgrade guide for the version you are upgrading to, and check the platform specific notes relevant to your platform.