I mean the file you want to edit is an XML file. Editing XML can be viewed in 2 ways from my experience; it can be easy and work perfectly after one edit, or it can be a nightmare after what appears to have been and easy and straightforward edit. There is no way to determine, so far as I know, which it will be at the start.
So to proceed, open rc.xml with your text editor. Click File > Save As >> "rc.xml.bak." You should now have 2 copies of the file, rc.xml and rc.xml.bak. ***The second one is your backup in case something goes horribly wrong; in which case, delete the 'bad' file and rename rc.xml.bak to rc.xml will put you back to your start.***
Next edit rc.xml as you read on the Openbox wiki. Test the changes.