What are the best guides/resources for implementing Undo/Redo in an app?

Game Programming Patterns happens to have a good summary of the command pattern which also deals with undo/redo:

http://gameprogrammingpatterns.com/command.html

That's a very broad question that I imagine depends very much on the particulars of the application. A basic undo/redo approach is described in the Command pattern chapter of Design Patterns.