User story writing effective acceptance criteria

Write clear user stories with testable acceptance criteria. Format: 'As a [persona], I want [functionality], so that [benefit].' Example: 'As a returning customer, I want to save my payment information, so that I can checkout faster on future purchases.' Acceptance criteria using Given-When-Then: Given I'm a logged-in user, When I reach checkout with previously saved payment methods, Then I should see my saved cards as options, And I can select one with a single click, And the form auto-fills payment details. Include edge cases: expired cards, declined payments, first-time users. Definition of Ready: story has clear acceptance criteria, designs attached, effort estimated, dependencies identified. Definition of Done: feature tested, documented, deployed, analytics tracking added.