I would also appreciate recommendations for experimental languages with interesting syntactic/semantic properties that I can explore.
1. A desire to learn it. Learn to shape that desire. Like I want to go to Colombia to talk to hot girls, etc. It can be a straight grind at times, so you want to feel like it's gonna be worth it.
2. Start learning phrases you can use from day 1. Pimsleur is good because it teaches you almost perfect pronunciation from the beginning. Pronunciation is more important in some languages than others, but most importantly it gets you in the mindset/vibe of that language.
3. Structured lessons. Duolingo is alright just to start. Assimil is good once you're around A2 or higher. It would be good, as another person mentioned, to learn the general grammatical structure. You don't need to be a wiz, but you will have enough to infer the correct way to construct what you want to say.
4. Natural exposure. Go to a language meetup, have a regular Skype tutoring session twice or 3 times a week. Or of course go live in the country. This makes speaking it part of your life.
No. Schools follow this method. I doubt anyone acquired a language in school.
I have acquired 3~4 languages, having practically nothing in common, before ever going to school at age six. (Kabyle, French, Algerian, and Arabic. Algerian is a mix of Arabic, Kabyle, French, Spanish, Italian, Turkish, and Arabic but with a structure closer to Hebrew than to Modern Standard Arabic, and is unintelligible in the "Arab world")
What has worked for me is content consumption. Reading/watching/listening to content in my target language about topics that interest me. I will read about whatever I usually read about, just in my target language.
Language acquisition can be tedious, doing it through topics that do not interest you is a sure way to give up.
Languages (apart maybe from Esperanto) are not designed but evolve organically.
1) Grammar tables. You don't have to memorize them, but getting "Pareto proficiency" with all the grammar concepts makes everything go so much easier. Tae Kim's guide for Japanese is the holy grail of this, despite having some minor issues. A good textbook serves the same purpose but you have to skip the fluff
2) Reading, news articles to books, social media, etc. The more personally interesting, the better. Look up every word.
3) Conversation, even if its just other learners, for me once I use a word in a conversation it never leaves me
Things that didn't seem effective for me: listening (sorry Pimsleur), Anki (never sticks), practice/review sections of textbooks