Hello, friend.
It can go wherever you want it to go, remember that for a current to flow, there must be a complete circuit. For organization's sake it is generally put at the very beginning of a circuit but this is not always possible, if you want to control different lights etc. As far as the physical location. I would recommend a small switch on the back, where the CD tray used to be.
Can you post a link to that diagram you're talking about?