What is the best way to learn a new language as an adult?

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.

The few points I'd bring up are:

1. If you want to reach a high level of proficiency you should basically be in love with the language. If you're forcing yourself to do something, learning it won't come as easily.
2. You should use a diversity of tactics, experiment, and find what works best for you.
3. Comprehensible input is a very good idea. There are different standards for what makes for the best comprehensible input, but I would say you should focus on finding songs, books, comics, shows and movies etc where you can still get something out of them even if you don't understand everything, and beyond that learn not to expect to understand everything. Being around L1s can also be very helpful, but it depends on how you make use of their input.
4. Define what you actually want to get out of your language learning by setting realistic goals. If you want to learn a new language because you hear it makes you less likely to get dementia later in life, then you might prefer a more game-y or puzzle-y approach. If you're interested in translating into your first language, then focus on understanding input more than generating output. And so forth.

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.

Being forced to use it with natives is the most salient answer