Designing the architecture for a game

I'm currently reading "Game Programming Patterns by Robert Nystrom". Basically, the book is about patterns to help you to design the architecture of a piece of software (In my case a game).

After reading a few chapters and wanting to finally try to make my own little game, I've run into a problem: I don't even know HOW do you design the architecture for a game. I hacked together some tiny games many times in the past, but I never really finished one of those, and never wrote done the design for it in the first place.

I guess that whole designing can be split into multiple parts: First the basic description, like as if you were explaining the game to a friend. At the end probably comes a very detailed "explanation", essentially consisting of a list of classes and subclasses, and maybe some little notes on how they work together. But I have not the slightiest idea what comes in between those two steps. I mean, the first one is easy, but the last one is presumably very hard to do without intermediate step.

So my question is: How do you get from a basic explanation to a detailed class-architecture (Which then is used for the implementation)?

Kind regards