The application has help sections for any of the rules todo.txt format follows, but you can use as many or as little of them as you want.
I wanted to go through in this blogpost about the process, or what I remember of it anyway. I've always just fixed bugs in Plasma or Frameworks or even in apps like Dolphin, but never made anything from scratch. So I thought that writing this down might be helpful for someone in similar position.
Starting a Qt project using KDE libraries, especially QtQuick, is not that difficult in the end. KDE has really good documentation for starting up. I mostly followed this guide to get started: https://develop.kde.org/docs/getting-started/kirigami/setup-cpp/
Working on the application was not that bad either. I've worked on various C++ and QML files during my work on KDE software, so I had easy time getting into the flow.
I think the most difficult part for me was the CMake files: I couldn't really understand them or what they do, I mostly followed what other projects did and replicated them.