There are several guides and tutorials to help you study testing. But they only help you to learn syntax and some other basic knowledges, not the real problem-solving, or high-level. This guide is for the developers who wanna be seniors.
There is a good way to make test scenarios. Most of the time, it depends on the requirements, but once you scope all of the todos and items, you can make scenario depending on that. The workflow of the project is the test scenarios, and todos and items are the each test cases. This will help you how to make test scenarios and cases, step by step, and let you do it by yourself.
This sample guide uses node.js to build server-side application and restful apis, and react-testing library. Not so bit app, just a simple. But working with here, you can learn every single detail of testing that can be risen in the real world, step by step.