How do you form test design if there are no test cases yet?

If there are no test cases yet, I form the test design as follows:

1. Requirements and specifications analysis — studying functional and non-functional requirements of the system.
2. Defining testing areas — identifying key functions, usage scenarios, boundary conditions.
3. Developing test scenarios — describing sequences of actions, expected results.
4. Prioritizing tests — highlighting critical and frequently used scenarios for primary testing.
5. Choosing testing methods — functional, integration, load testing, etc.

For example, for a web application, I first test the main user flows (registration, login, main operations), then add negative scenarios and check for error resilience.