Before you install any patch , it would be good to know what you have already installed on your system.
To find which pathes have already been applied do :
showrev -p
Then download the needed patches
unzip or untar them in any temporary directory
(could be /var/spool/patch) and do what we have to do for every patch, go thru the README to see the prerequisites, disk space requirements , etc anf follow the instructions on the screen when you run install_patch or install_cluster.
It is very straighforward and easy. And nowwadays, it rarely bombs.
Dont worry, in case of problems after applying the patch, you can always do a backout unless you specified no backup during the install of the patch.