I would first gather all the requirement and collbrated with pm and dev team for any open questions. Then I would define scope of testing by identifying whats in scope and out of scope The test plan would include all the test type, including functional and non-functiona, along with environments where tests needs to be executed. I would also scope what automation tools we need to use Additionally the plan outlines the risks & mitigations, time lines, test milestones and responsibilities to ensure team can execute test strategy effectively