The Online Store From One Sentence
Work from the brief alone. Write what you would produce — actors, requirements, data, a first slice, your unknowns — before opening anything below it; the exercise stops working if you read the reality first.
"Build an online store." Run the whole loop: frame, actors, requirements, constraints, invariants, assumptions, unknowns, decomposition, first slice, data, interfaces, state, failure modes, implementation order — as one page per stage, each stage only as far as the previous one justifies. Stop where the requirements stop.
Your attempt
Nothing here is checked. It exists so that the reveal below is a comparison rather than a reading.
Writing the twenty-page specification. Every stage is done thoroughly, the document is complete, and it took the two weeks the first three slices would have taken — during which the payment provider's webhook requirement, which the first payment slice would have found on day four, is still a paragraph nobody has tested.
Read this even if you are confident. It is here rather than behind a button because it is the move most people actually make, it produces things that look like a project, and its cost arrives when the first hard requirement has nowhere to go.