They do it by becoming a position, assistant coach in college, after being a highschool HC, that is wildly successful.
Then they get a reputation as a wildly successful, awesome position coach, assistant coach in College. And because of that, then a NFL team, etc, then hires them as a position coach, assistant coach in NFL.
Then they are wildly successful as a position coach, assistant coach in NFL, and get promoted to assistant Offensive coordinator, assistant defensive coordinator, assistant coach, where again they are wildly successful.
Then after that they are made offensive coordinator or defensive coordinator, where again are wildly successful.
Then after that they are made assistant head coach/offensive coordinator or defensive coordinator/assistant coach, where again wildly successful.
Then they stay at, in that role, continuing to be wildly successful, until the head coach is fired, and then they get hired as the head coach about 6,7,8,9,10 years after being a highschool head coach, because of the fans, the GM, pres, vp, owner, etc.
That's how a highschool coach becomes a NFL HC in 10 years or less.
If, since Paul Wulf coached as a assistant assistant position, assistant assistant coach, either as a intern, or getting paid almost nothing, in the NFL, then a Highschool HC, can become a NFL HC in less then 10 years.