I started studying Spanish in the year 2000. I had a teach-yourself-Spanish textbook, i got through to the Lesson 12 (I think). Then I opened only the grammar section to check on grammar.
Everything else was immersion. I traveled a lot on business throughout LatAm and was always practicing. For some time I even had a novia/friend in Argentina. When i check back those emails from early 2000's I have to laugh... but it was as going step by step, laying brick upon brick, the language skills grew... When I lived back in the USA I watched Univision a lot, at least one novela per day. When I was not home, TiVo was a good friend. I also read Spanish books - John Grisham. First I read in Spanish the book I already had read in English before. Then, brand-new-never-read-before novels in Spanish. And speaking, speaking, speaking. We had several Latino maintenance guys in apartment complex I used to live in, so I practiced Spanish with them. When I traveled on business within LatAm, I always spoke Spanish. Sometimes it was funny, having a small-talk conversation there, Mexicans speaking English to me practicing it, and I speaking Spanish to them practicing Spanish. Now, I am 99.9% proficient in Spanish. I even write my own Spanish contracts, to which local lawyers make very very few corrections. I draft and design my own promotional materials, brochures and presentations in Spanish. Again, very few corrections made during proofreading by the native speaker (who studied journalism btw., so has very high --above-population-average-- language skills).
What I want to say, the same way I learned English, I learned Spanish. You have to keep practicing the language. For example, I lived 6 years in the US. I almost did not have accent. Now, I have some accent, because I am no longer in the environment. To keep English up, I watch TV, read books, and speak on the phone ... for Spanish, I live in the Environment, but still watch TV, read books in Spanish as well (last one - Las Munecas de La Mafia!! I recommend it 1000%!!! then the whole Zobeida and Agosto thing will make much more sense,a nd also El Cartel de los Sapos - not the TV version, the book!).
My aunt who was a language teacher in Prague once told me to read the books in foreign language (you have to have some vocabulary level for that) but do not stop on unknown words to consult with the dictionary. Keep on reading. Sooner or later, the word will become "known" to you from various context-use throughout the text. If, after you "almost" think-you-know what an unknown word means, then double check the dictionary to get it 100% right. When I started to take my English-language skills on the "next level" I read "Birds of Prey" (my very first book read in English) in about week's time, and consulted the dictionary about 10 times. First half of the book I was understanding maybe half of what I read. By the end of the book, I got 95%. On my second book, I practically understood the whole novel without any problems.
To add to that, I watched TV in English (in my country the broadcast channels had dual language tracks) so I watched some US series with their original language track.
Then I moved to the US, and the immersion helped to "nail it down".
The same went on with my Spanish. Reading books, watching TV, carrying conversations in Spanish. As I said, it did not come overnight (February was a 10-year anniversary since I first took the Spanish textbook in my hands).
Also, important in the whole "language studies" is how "attracted" you are to the particular language in question. I studied German for 6 years when I was a very little kid (which should have made it very easy) but I do NOT speak German (I can understand SOME written text, but not much). I studied French for 4 years at high school, but cannot even "blink" in French... simply, those languages were quite "foreign" to me.
P.S. I found out that reading a few novels in a foreign language (after you have built up a sufficient vocabulary, of course) makes so much difference. It absolutely takes your language skills on the next level. In other words, after you read and understand a novel in a foreign language, you are "advanced". Then you may start dreaming in foreign language, and that means you "made it". That means, you are "thinking" in the foreign language, and not translating back-and-forth.