By exposing them to as much native speaking and writing as possible and having them produce as much speaking and writing as possible.
There really is no way around immersing yourself in the language you want to learn and actively working with it, if you want to acquire an advanced level of understanding, including knowledge of a word's or phrase's exact meaning and usage.
You can train certain parts of the language specifically, e.g. pronouns, or you can learn all of the language more slowly at the same time. It doesn't matter. What matters is spending time – and effort ! – on the language. And that effort doesn't have to be directed. Kids learn to speak without parents forcing them to learn the correct usage of all pronouns at a certain stage.