What's the best way to learn German ?

Like any other languages: LEARN it seriously: read / write ( grammar / conjugation / vocabulary. Watch videos in this language, practice it regularly, etc.. etc....

Once you're not a small child anymore?

Take classes from a good teacher. After that , immerse yourself: books, movies with good subtitles in your native language, music, spoken radio.

It's how I did

Go to a German bar on Game night.

Have a knack for it.

If there was one definitive way everyone would be using it.
You can tell by the wildly varying other answers that there is not.

To learn German may differ from person to person.
The best way to learn German is to hear it and practice speaking it every day in the context of your normal life. Study German vocabulary and grammar, important is  reading, writing, listening and speaking.You will find that German is such a beautiful language to learn. Give it a try.

Immerse yourself in the language by learning through textbook, audio/online program, and perhaps most importantly through some experiential learning like with a tutor who is a native German speaker. Also, natural settings where you can practice the language would be helpful.

Take a language course online to get the basics.

Move to Germany or live with a German Family.

Move to Germany.