Four phases. Each one ends with something you can hold, and each one is priced on its own so you can stop after any of them. This is the same shape every product on the shelf went through.
I write down the problem, the person with it, and what has to be true for this to work. You get a scope you can price and the riskiest assumption named out loud.
One real path end to end before I widen anything — real data, real auth, real deploy. Ugly and working beats broad and hypothetical.
In front of users while it's still cheap to change. Every product on the shelf shipped before it felt ready, and every one was better for it.
I run every product on this shelf myself, so I know what year three costs. I can keep yours running, or hand it over with the docs to run it yourself.