Things are very easy if you read the upgrade guide as everything is mentioned in the guide.
https://websmp208.sap-ag.de/~sapdownload/011000358700000333942010E/GUIDE_UX_ORA_UPGR_112_400.pdf
You need to have a separate file system /oracle/SID/11204 for 11G installation and install 11G database software and also apply the latest SBP.
You run the dbua.sap.sh from the /oracle/SID/11204/sap/ora_upgrade/post_upgrade location.
There are many other tasks involved before you start the upgrade, like setting the environment variables. Take time to read the guide.
Regards
RB