It’s easier but more laborious than it looks like. Think on how you learned your first language - you were considerably less smart than you are now, but if you’ve spent most of your time interacting with it.

A few major hints:

• Better is enemy of good. “Grammar of brokenness even convey meaning - when person has want for meaning”.
• Use it actively (writing, speaking) and passively (hearing, reading) at the same time.
• Pets are specially good to train your language with, as you know that they won’t judge you for your mistakes.
• If you’re playing scrabbles and words from the “wrong” language keep popping up, you know that you’re doing it right.