Mezzofanti studied languages all his life. There were no language tapes at his time and he did not attend many language classes.

Most of his learning was done in two ways:

• Reading books (translations in the target language of books he already knew, polyglot bibles, grammar books, dictionaries, etc...)
• Speaking with native speakers of his target language. There was always a large supply of speakers of the most remote languages in Rome as it was the world capital of Catholicism.  Sometimes Mezzofanti would go and speak with Hungarian soldiers to learn their language, or attend to the wounded in a hospital to learn czech or albanian. There are countless anectodes about how he picked up languages from people he just met.

Although some people would find it less challenging if I said Mezzofanti learned languages from a magic wand or in his sleep, his biography tells us that he had to study new languages like evererybody else. He certainly was more gifted than the average learner, but you should remember that this was almost the only pursuit in his life and that he studied language all his life. It seems quite clear that the more languages you already know, the easier it is to learn the next language.