The general steps are:
1. Install 11g home
2. run pre-upgrade script on old home
3. shutdown db on 10g instance
4. startup instance from new 11g home in upgrade mode as single instance
5. run upgrade script and recompile
6. update timezone
7. startup as normal
8. track and fix sql performance that changed
The details are depends on what are your requirements. This is the best presentation around:
Ed is correct in term of normal upgrade. However, I believe this is a RAC system, so you will need to upgrade your CRS before you can upgrade the database.
You can do in-place upgrade by select "Upgrade" from Universal Installer, but for me, 11gR2 is significant changes so I would recommend out-of-place upgrade by building the new 11g clusters and migrate your data over.
Hope that help,