If you are struggling with concrete enough acceptance criteria, acceptance criteria that doesn’t specify the “how”, just the “what”, one technique that I find especially helpful is specifying “How will we demo?” to the story. This usually puts the product owner and the team in the right frame of mind for what we’re getting at with acceptance criteria something both specific and actionable. For instance, if you are designing a shopping cart, the question “How will we demo this” may bring up a discussion of one book from the catalogue being searched for and added to a shopping cart and displayed for view. Typically I’ll have this as part of the template for a story card. Part of the beauty of this method lies in the act of imagining what will count as done in the future. There is something about the act of forcing humans to “remember the future”, of what the screen or web service will look like in two weeks during the iteration demo, that helps substantially when trying to get enough detail to estimate or commit to a story.