Not that I had any success with it, but here is a book/site about how to learn which I found interesting:
https://www.fluentin3months.com
The theory is to start from day 1 with 15 minute real conversations with native speakers (Skype pals). You either pay them or offer them the same in English afterwards. You are not permitted to fall back to another language during the call! This makes you study damn hard between sessions to improve you vocabulary and reduce the awkwardness. Benny (the author) spent months working in Spain and learning no Spanish, and stumbled on this method as a last resort.
It is very similar in spirit (immersion, but not requing presence in a native speaking country) to the Kato Lomb approach:
https://en.m.wikipedia.org/wiki/Kató_Lomb
Which is documented in her book "Polyglot". (Cut off from native speakers she used romance novels, which have a very low reading age, dictionaries of the language itself, rather than translation ones, and radio broadcasts, and was so successful that the Russians arrested her as a spy when they took Hungary in WW2, and she addressed a soldier in perfect Russian.)