What do we need, to compare two historic events?

If we look at the works on the Thirty Years War for example, the way up to it, there is a specific event (the beginning of the Jülich-Cleve succession crisis and it's 'wars' in 1609 and 1614) that is sometimes viewed as of great importance to the beginning of the Thirty Years War and as an event that foreshadowed the beginning of the said war in the Bohemian Uprising. There is on the other hand works denying this connection (I can cite German and English sources for these claims, but I do not think they are necessary for my question, since it is less about a specific event and more about methodology; but if they are needed for an answer I will add them). The gist of the incomparability appears to lie in the different interpretations of the Thirty Years war, so the problem isn't the succession crisis, but the premises regarding the interpretation of the thing it is compared to.

While there is enough discussion here already, on if these things are comparable, and how that should be done, there are more than enough examples where I would intuitively say 'no reasonable comparison can be made here' - examples would be comparing the Ancient Roman Empire to the British Empire or saying 'this 20th century person is acting just like this 16th century person'.

So it all boils down to my beginning question: what do we need, call it a list of methodological steps, to compare two historic things?