How to add a TabBar to NavigationController based iPhone app

I have a simple NavigationController based app. The main window shows a TableView and selecting an item loads a sub-view. I used Interface Builder for UI views.

Now I want to add a TabBar to the app. Where do I put it? Do I need a TabBarController? Should it go in the MainWindow.xib, or RootViewController.xib?

How do I tie it together with my NavigationController?