In a class diagram, how do I know what methods to add? I’m not sure what the connection is between methods and other (i.e. Activity and User Case) diagrams. My guess is that methods can come from Sequence Diagrams, but then I’m not sure how sequence diagrams relate to the Activity and User Case diagrams. I’m trying to understand how all the design diagrams fit together. How do diagrams connect or flow into each other? I’m presuming they’re related.
The Use Case and Activity diagrams are high level things showing how the users (Actors) are going to interact with the system. At this stage there will be no classes or methods.
Class Diagrams and Sequence Diagrams are the details of what's going on in your system to implement the high level stuff.
You can't really have a sequence diagram without the methods. But drawing one might reveal that you are missing some methods that you need to create.
Which methods should you add? The ones you need! You are allowed to simplify diagrams by leaving bits out. But methods are things that you realise that you need when you start thinking how the classes will interact with each other.
Ultimately, UML doesn't do the design work for you. It gives you a way to illustrate the design in pictures, not (just) text in a design document.
In a class diagram, how do I know what class methods to add?
That depends entirely on why you are creating the diagrams in the first place.
If you are creating the diagram with the purpose to generate code from it, then you need to add all the methods of the class to it. Otherwise, the code generator doesn't know it needs to generate those methods.
If you are creating the diagram to explain something from your design or your code to someone else, then you should only add the details (including methods) that are needed to get the point you want to maker across.