How to design effective test cases in software testing?

[scrollsense]

Hello everyone in the Squashtest forum,
I am currently studying software testing and using the Squash tool to manage test cases. However, I am still quite confused about how to design test cases in the most complete and effective way. I would like to ask experienced people where to start writing test cases? Should I stick 100% to the specification document or do I need to add a more detailed business analysis step? In addition, for Agile projects, how detailed should test cases be to ensure coverage while still being flexible? I hope to receive practical sharing from everyone so that I can learn and improve my skills. Thank you very much!