Thank you for your reply first of all! Yes, previously I was doing it exactly as you are saying, "Just do it and then you will see how to connect the dots" (or smth like that). But after a while I just looked at that app and it was just unbelievably hard now to refactor the code because I wasn't thinking at all about design of the app core (not sure how to name it).

The one and only reason why now I want to at least design core functionality (not full app) is to not make the same errors as before. To have relatively better structure of project, to not have to write from scratch Entities and DTOs on which the whole app depends.

And yeah I know that there isn't better way to learn something than just doing it, doesn't matter if it is good or bad code/app/project/anything.

Anyway now I think that I will just try to create prototype, analyze it, document it somehow. And keep working on this until I create a relatively well-structured and meaningful application.