Tester is involved from the inception of the idea
Why - They will be in a better position to review the product and find bugs if they know about the management's, designer's, and PO's vision of the project.
Tester should ask questions even if they are basic. Usually bugs hide behind the basics. Here the expectation is not that the tester will have revolutionary ideas about how this feature should be created but more that they will get an understading of what is being built.
Writing acceptance criteria
Tester should write the acceptance criteria before planning, just after the story cards are written.
Why - Acceptance criteria clarifies the story for the developer and tester. Acceptance criteria should also form a basis for automated and manual tests. Any issues found that were not covered in the acceptance criteria should be a new card and not a bug.
Acceptance criteria should be approved by POs before they are provided to the team. Once the PO is happy with them, the acceptance criteria is attached to the respective story cards.
Backlog grooming
Acceptance criteria is read to the team. Developers can suggest ammendments to the acceptance criteria. Objective of this meeting is to finalise acceptance criteria.
Why - This meeting ensures that planning meeting are short and are just about estimating. Otherwise planning meetings are stretched longer because of discussions that now happen in backlog grooming.
Planning meeting
Estimate development effort and testing effort for each story card. Testing time and difficulty should be taken into account when estimating for planning.
Why - There have been a few times where something is easy to develop but hard to test, or manual testing is needed or preferred over the cost of maintaining automated tests. Testability of stories should be discussed.
Post development
Run automated tests to ensure there are no regressions introduced during development.
Why - Developers run the tests everytime they make a change, still there have been times when I have found tests not passing. Even if people choose to ignore some faliures it is important for the tester to know what is not working. Tests that fail intermittently should be investigated by the devs.
Testers check that there are automated tests for all non-UI based acceptance criteria.
They also perform manual testing for the story card.