This method promotes self-teaching by immersing with input, which is language produced by native speakers. We pair this with a tool I'll explain later called an SRS, for memorizing info.
Learning by consuming input is essentially the cornerstone of this method and everything else is consequential of that, so let me really drive this point home. We follow a process of naturally picking up a language, similarly to how you did as a child for your native language but enhanced by the agency that comes from being an adult with context of the world. Many call this "language acquisition" instead of "language learning." So you won't deep-dive into textbooks to learn language from an analytical perspective. You won't formally study and dissect complex grammar rules, nor will you memorize vocabulary lists or try to figure out how to say things by stringing these vocabulary words and grammar rules together. You will "just" consume native input until you understand your Target Language well.
If you already have a history with your Target Language, you can probably skip to acting on the "Beginner-Intermediate or Intermediate phase" of the guide - however it is still important to read the "Absolute Beginner phase."
Keep in mind, everybody is different, and so as far as immersion methods go, what will be exactly most efficient for you may vary. Part of the fun of the journey, in fact, is figuring these things out for yourself. However, this is basically the best compact starter guide I can come up with packed with enough advice on exactly what you should do to avoid common beginner pitfalls.
SRS stands for Spaced-Repetition Software which is a tool we use to commit information to memory. You can think of them as flashcard programs. It will force you to indefinitely review flashcards you put into it, but gradually less and less frequently over time, as long as you answer correctly (it is content-agnostic and you grade yourself).
There are multiple options but I recommend Anki. Compared to other options like Quizlet, Anki's scheduling algorithm is superior for optimizing your retention/memory with the lowest effort required.
In the SRS, you will make only one kind of flashcard: sentence cards, which show a full sentence on the front. When reviewing these flashcards, you test your ability to understand the sentence.
This method is very anti-textbook and pro-self-teaching, but the unfortunate reality is that you need some sort of bootstrap to get started. Ideally, you will do this with a basic grammar book. Alternatively you can do this with a "premade deck" in Anki, but in the interest of keeping this guide short, I will just say you should really try to avoid that. Here is the full explanation if you want to know.
If you're going to use a grammar book, use one that gives you example sentences, not one that asks you to fill them out. This is important because as a beginner you can't invent input!
Add new sentences and phrases from your grammar book to your SRS. Rate your reviews based on if you completely understood the sentence or not - though there are 4 different ratings in Anki, most users stick to only pressing "Again" for fail and "Good" for pass, which I recommend. Don't bother adding sentences that don't actually teach you anything new, just focus on getting sentences that teach you new grammar or new words.
If you need recommendations for grammar books: I have heard good things about All About Particles for Japanese.
Even from the beginning, you can simply watch TV or movies in your Target Language, with no subtitles. This may seem pointless because you won't really understand anything at all, but it's very important for developing your listening ability. A lot of your progress here happens subconciously. I would say a good minimum is 30 minutes of this per day, with 60 minutes or more being ideal, but do as much or as little as you want. It's important to not burn yourself out pushing yourself beyond what's comfortable.
Listen to audio passively as much as possible. This "passive" listening targets times when you're busy with real life and can't "actively" immerse, like when you're at work, cleaning, doing homework, etc. For example, listen to podcasts (for-natives podcasts, not for-learners), or just play TV shows in the background without watching them.
At this stage you should have finished your grammar book or premade deck (either you finished them completely, or you got bored of them and decided to move on to the next step).
From now on, you will only learn from immersion.
Now, you can take ("mine") sentences from your immersion and put them into your SRS as a flashcard. This is how you will memorize new vocabulary and grammar you come across. Sentence mining is the second most important part of this approach (after immersion).
In this process, you're looking for a specific kind of sentence: a 1T sentence, which is a sentence that teaches you one new piece of information, usually a new word but sometimes a new grammar point. If a sentence doesn't contain any new words or grammar, it's a 0T, two new words is a 2T, etc.
For an example of a 1T, if you were learning English, and came across a sentence like "We have not made much progress yet" and, at the point in time that you read it, you knew every word except "progress" - you would make a flashcard with the sentence on the front, and the meaning of "progress" on the back.
You should mainly be making cards from 1T sentences. Never make a flashcard from a 0T sentence as there's no point since you're not memorizing anything new. 2T are occasionally acceptable but you really should stick to 1T as much as possible.
Aim to make 10-20 of these per day depending on your appetite. I would recommend starting off with 10 per day and re-evaluating after 2 or 3 months.
At this point, it is acceptable to turn on Target Language subtitles for TV shows or movies you watch, in order to be able to read every word and mine efficiently. However once you're done mining for the day, you should turn them off again.
When TV shows or movies become harder to mine from (for example, you are starting to mostly understand them and are coming across 1T less often), you can start to mine from reading books (more below).
Read content by-natives and for-natives, even if it seems "above your level" (if you just start reading them anyway, eventually they will be your level!). Look up words you don't know (you can choose to look up every word, or only every few words, whatever you feel like - just avoid burnout, as looking words up is also tedious).
Try to understand everything you come across, but if you get stuck on something, don't waste time getting super hung up on it. Just move on to get more input.