Diagram or Storybook an Existing App

I have an app that was developed some years ago. I have done a little search of Google but cannot find the correct process one would follow to diagram the various parts of the app. Any suggestions how this can be done with a plugin for xCode?

So what you mean is that you want to break it down and create a template that represents the structure of the App. Is that correct?

Chris that is correct. I find that having a diagram showing the interconnection is easier to grasp.

Without knowing what the App looks like my suggestion would be to use it and observe how many screens (or pages) there are and what you have to do to get to those screens. If the App is a tabbed view then how many tabs there are and what the layout is.
Just start drawing on paper showing the links you perceive there to be between each screen.
Look at what data is shown and see if you can come up with a data model.