> Firstly, I should say, there's no "formula" here.

There are a lot of variables, and luck does play a big role. But with that said, there is something very akin to a "formula" at the high level. That being Steve Blank's "Customer Development" methodology, as laid out in The Four Steps to the Epiphany.[1]

[1]: https://www.amazon.com/Four-Steps-Epiphany-Steve-Blank/dp/09...