The best methods are:

1. Go to a country which speaks the language you want to learn
2. Don’t hang out with people who speak your native language
3. Live with a family (often called a host family) and spend time with them
4. Study a lot
5. Stop watching movies in your native language and instead only watch in the language you want to learn. Once your vocabulary is built a little only use that language’s subtitles and stop using English
6. Get a girlfriend or boyfriend who speaks the language you want to learn
7. Google for language schools in the country you’d like to go to (South American and Central American schools for Spanish can be really affordable)
8. Use Duolingo to build vocabulary (you won’t become fluent from Duolingo, you need to use it as a supplement to the other methods)
9. Fall in love with some bands that sing in the language you want to learn and listen to them regularly (YouTube is a great way to learn of those bands)
10. Once you get a base start trying to read and translate children’s books and progressively up the reading level as you get better

I too confirm that this a great method. Also, it can be super funny when there are miscommunications between the two languages.