[segh]

I don't know if it's "simple", but there is a well established corpus of knowledge, as taught by Y Combinator and others. He describes a fairly standard path.

First he has domain experience (teaching French and English in Korea). This experience exposes him to a problem (students know grammar but can't speak). He then explores solutions to this problem. First with a blog and a book (sells enough to "cover the bills"). Then he iterates to an interactive book (which now was "selling well"), and then finally moves on to build the fully fledged webapp. And with that, out of the 5 billion internet users, he can convince 800 of them to hand over their cash.

Of course the story is probably streamlined and leaves out the dead ends. You can only connect the dots in retrospect, as Steve Jobs said.