Immersion. Learn as many random words as you can, by pure memorisation, and then sit around people who speak the language so you can force yourself to learn the language.

I 'learned' 2 languages in school and by far the most important part is knowing as many words as possible, and actually speaking it with natives. Other than that, you wont get too much out of written exercises or duolingo or even books. Just stick to those two things.

However, the best way to learn random words is to play a game or read a book, and then google / dictionary search any word you don't know.

It should be easy to find people who can speak the language with you, but if you can't find anyone, well... why learn it?

I don't know about any research on the matter, but I can say 2 things that worked very well for me:

1) take an intensive course first

2) then live in a country where the main language was the one I learned

Granted, point 2 isn't easy, but nothing can beat it. The most important point is to use the language on a regular basis. If you don't, you'll forget it quickly.

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In my opinion, Assimil for learning the grammar and Pimsleur for speaking and listening comprehension. That’s how I learned German by myself enough in a year to get a job in Germany

More important than the method, though, is that you never miss a day of practice

Move to somewhere they speak it and force yourself to use it. Even if you could get away with using a language you know better. Accepting that you would get it wrong, and keep trying even if you sound stupid and it feels hard would also help.

Keeping up motivation seems most important to me. After thinking about it for a while, it seems nearly impossible to learn a second language unless you have some interest in the culture behind it.

Be very young and grow up in a country where the new language is spoken.