Then it depends what is a "short period of time" for you and what is "fluent" for you. And what is a good method for someone may not be a good method for you... But being immersed in the language is definitely a great plus.
The way I learned English was simply by trying to think in English instead of French (my native language). Whenever I didn't know how to say a word, I'd look it up. I'd also watch movies and TV shows in English. And I'd have many many, many penpals. It was the most important I think, having penpals. Because language is about communication after all.
I wouldn't say it was the fastest way, but unless you really have a gift for it, you can't learn a language fast. Being able to communicate fairly well in a foreign language without requiring the help of a dictionary takes at least one year.
You need to practice a lot before you actually achieve fluency, that is, being able to express yourself in a way that people think it is your native language.