Custom renderer for placing button below TabbedPage

Hello
I'd like to write a custom renderer for Android and iOS which places a button on a Tabbed Page below the page content (the button should be visible on every page). Could you give me some help because I don't know where to start (which methods do I have to override in the custom renderer? How to add an additional element in the customer renderer?
It would be great if someone of you could give me some help for the start
Thank you very much