2 resources from Getting Real (37signals) we point founders to, and the questions each answers.
✍️ Essay
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Why we picked it
The healthy counterweight to over documenting. 37signals argues that a thick spec creates false agreement (everyone reads the same words differently) and locks you in before you have learned anything. Their fix maps onto our answer perfectly: write a short one page story in plain language, then move to rough sketches and real screens. It frees you from thinking a good spec has to be long.
Why we picked it
This backs up the sketch the flows part of our answer. Design the screens before anyone writes code, because a paper sketch is cheap to change and code is the most expensive thing to change. For a non technical founder that is empowering: the interface is the part you can actually own and communicate, and it gives the developer a concrete target instead of abstract requirements.