The answer is no secret but it's hard to achieve. The fastest way to learn a new language fluently is to speak it, constantly. Yes, it's easier for children to acquire a new language but they learn it because they are constantly forced to use it. I've known a number of people that have lived in a foreign country yet they did not learn the language. Why? Because they could get away with using English most of the time.…
Of all the language learning methods I've seen I've only seen one thing consistent across all of them that in the end leads to results and that is you have to legitimately feel that it's deeply important to your life story to actually learn it to fluency. At some level you actually just have to care about the outcome. It's an emotional switch that once flipped, actually the method is irrelevant. I've seen every which way work. The constant was that it mattered to those people. And no one knows how to turn that switch on/off. Therefore I agree with the sentiment no one knows how to do it. I guess I'd be more specific and say that no one knows a fool proof method to engineer the outcome because it relies on emotional states and belief systems. As for the intellectual understanding of how to execute against learning a language. Plenty of seasoned language learners know full well what works and what doesn't.