1. Be very young
2. Be so young that you don’t understand you’re learning a language, you’re just making sounds with grandma
3. Be exposed to unique sounds like the German “ü”, the French “r” and the Dutch “ch” and try to imitate them when you’re 3 years old and your brain, tongue and throat are still flexible

If you’ve fucked up 1 to 3, plug away at it for a long time, then at some point, before you think you’re ready, live somewhere where you’ll have no choice but to use that language.