The best is to move to that country and start working there an you will be forced to learn it.

Probably not you want to hear, but being around native speakers

Immersion. But the Seedlang app is pretty good.

Input is king. Anki is good for memorization.

I learned quite a lot of Norwegian by just beig in Norway for three months. I especially enjoyed watching television with the subtitles for the hearing impared turned on.

Another option that the pros (like politicians etc.) like a lot: Nonnen van Vught.