It's broken us into the key sections - details, a visual tree of the structure, the logic tree and relationships, and, finally the source code. It's a pretty simple process of creating the menu and appending the elements onto it, much like a normal drop down menu. The key is in the set_submenu function, creating a new object for the code to append the submenu elements to. The rest is just simple appends.

They've also included a screenshot to illustrate the end result.