For yet another data point, my brother has learned a number of languages. His best language is Mandarin, which he started learning when he was 19. He speaks, reads, and writes at a college level, with no discernible accent. He has varying levels of fluency in a number of other languages including French, Thai, Indonesian and Japanese.
His method of learning a new language is to just start out memorizing vocabulary. He is able to achieve a sustained acquisition of about 100 new words per day. Once he has sufficient vocabulary he adds in grammar practice, sample conversations, etc.