depends
on whether you are patching in the same release or
upgrading to a higher version.
If you are patching, download the patchsets.
Every patshset has its own readme doc. go trhough it.
Mostly
1. do a clean shutdown
2. take a complete backup
3. install  the patch.
thats it!.

if you are upgrading..u can use ODMA or export/import 
or ugrade scripts