BDSM approach. Do you really understand your project?

When we write test documentation for our project, prioritise specific tasks or even start testing a new application, one could ask the question: where to start? The functionality of the applications is growing and becoming more complex, and it is increasingly difficult to separate the main and the secondary things, apples and oranges. How to most effectively find the primary goal of the project, study it, what main functionality to allocate, write in the documentation and finally test it - my BDSM approach will tell about it.