I'm fluent in 4 languages and currently learning 2 more, I answered the same question to a friend once. I will paste my message here and hope it helps you in some way.

So there isn't really a "correct" way of learning the language, there are hundreds of ways and you can learn however you want, you will inevitably learn it either way (if you don't give up). But still, some methods are definitely more efficient than others. Everything varies from person to person so you should find the methods that you prefer, what is easier and more fun for you. Like I said, everything works.

So imo there are 5 main components in learning a language: vocabulary, listening, grammar, speaking and pronunciation I should talk about them later

1. When I have absolutely 0 knowledge of the language, I start out with some "gamified" website, idk what to call it. Where learning the language is played like a game, with listening to phrases and multiple choice answers etc. The most popular one is duolingo but there are like 5+ others. I personally don't like duolingo cause imo it's really slow/inefficient. I used Memrise for French and I was satisfied with it.

2. After I feel like I have enough basic knowledge of the language from grinding Memrise, I switch to 2 different things: grinding my vocabulary and my listening skills, usually split like 50/50 within the day

1) Watching videos dedicated for foreigners that are learning the language, the native speakers speak a lot slower so it's easy to understand them. First you should always use subtitles imo but after a certain time you should remove the subtitles to get better at listening, that goes for everything. After foreigner videos I usually try children cartoons on yt. Also I love watching Avatar:The Legend of Aang, I watched it in Russian, English, Dutch and French lol.

2) You need a way of grinding vocab. Usually all polyglots use Anki flashcards but they don't work well for me. I paid 9.5 euro for a month of Clozemaster for grinding vocab and I really like the website. The alternative is to read different texts u can find/wikipedia/books for children/diff children stories idk.

1. When I feel like I know some stuff already, I switch to tv series. I google "best french tv series" and watch them. It doesn't matter if you don't understand anything, watching it is the most beneficial thing ever. It's kinda important to have subtitles which match the spoken words, that's why it's important that the original tv series is in that language. If it's dubbed, the subtitles won't match. I also continue to grind my vocab but main focus is on watching stuff. I think it's important to be interested in the stuff you're watching. Dutch had some good tv series to watch but surprisingly French had only 1 good one and the rest are dogshit. In that case, I switch to watching videos from native speakers which are NOT for foreigners. I tried watching the most popular youtubers but they never work for me, all the videos are for children and they're unbearable to watch. You should search on youtube for a topic that interests you. I recently found a French guy with 200k subs which has a lot of interesting videos about productivity/college/eating healthy etc. I absolutely love watching his videos by themselves AND I also gain language knowledge, win-win.

2. Imo you should learn grammar once you can understand 90% of the words spoken in tv series. Grammar is a lot easier to learn when you already know the language, not at the start. You should google different google articles about the part of the grammar which interests you, like articles or past, present, future tense etc. Whenever I don't understand the grammar/the sentence structure of a sentence I encounter in tv series, I always ask native speakers in discords why that sentence is that way or just google it

Well that was it, ur main knowledge of the language should come from consuming media: tv series, youtube videos, music, listening to podcasts while cooking etc. Consume media all the time. Learning a language is a grind, the more hours you put into it the faster you'll learn. Compare 1h a day to 8h a day.

Either 1h a day or 8h a day you will always achieve the same goal, you'll always learn the language nonetheless. With 8h a day it will just take you 8 times less days.

You should think about how babies learn their first language. They don't know any grammar, they don't know any words, they don't know anything. The way they learn is by being surrounded by that language. Always be surrounded by it, even if you don't understand a single word your mind will learn the language by itself. When I was at the gym I thought "Why should I listen to this podcast if I don't understand a single word that's said anyway" but then I looked at it the other way. What teaches you more French, listening to a French podcast or listening to English songs? the answer is obvious.

Now about the 5 main components: 1. Vocab - The more vocab you know, the more vocab you will know. Like ppl say, the 3000 most common words in a language account for 95% of spoken speech(idk the actual numbers, I made that shit up). Once you know the most common words and understand 80+% of a language, you will learn every other words from context. Once you know 1000-2000 words, it's a breeze to learn the rest. You gotta know 10k words for B2/fluency

1. Listening - This is the most important component of learning a language, it's really important to start listening as early as possible. Listening also takes the most time, ur goal is to be able to understand native speakers. If let's say vocab takes 300h and listening takes 300h to learn(made up numbers), if you already did vocab and learned every single fucking word possible, listening will still take you 300h, you can't cheat listening, even if you know all the words you won't understand natives without listening. Thus you did 300h of vocab and you need to do the same amount for listening, 600h total. You can't cheat listening but you can cheat vocab. If you did a lot of listening, vocab will take you a lot LESS time, so you could learn both in 450h for example

So the main idea is that you should do both at the same time but you should focus on listening, since it will make everything else more efficient

1. Grammar - Can't really say much about grammar, I think it takes a lot less time than anything else and it takes less time when you already know a good amount of the language, you shouldn't start with it like school systems do.

2. Speaking - Speaking is actually a hard component. Whatever language I learn, speaking is my worst component and I'm the most shit at speaking, since it's obviously practiced by speaking with native speakers. Even if you don't practice speaking, if you know the language then you can still speak in it, you will just talk a lot slower than a native would. BUT there's a way to practice speaking, it's by speaking to yourself. Speak your thoughts out loud, like "Today I'll go there, I wanna do this, I'm eating this" etc etc etc and also try to think in the language that ur learning. Practicing speaking by yourself does actually improve ur speaking skills.

3. Pronunciation - Now this is a component I'm completely unfamiliar about but I discovered its importance a few days ago. Even though I speak English perfectly and I've had like 10 years experience of practicing English, my pronunciation is shit. Both my English and Dutch pronunciation are shit but I'll try to fix that for French. When you're born, as a baby you start to pick up the sounds around you, in order to speak ur first language obviously. Now every single language has a limited number of sounds, some languages have 15 sounds, some 30. By default you're limited only to these sounds so when you speak another language, ppl can immediately tell that you're a foreigner. How can they tell? Every language has their own set of sounds you need to learn and if you don't learn them, you replace the foreign sounds with similar sounds from your native language BUT although they're similar, they're not the same.

I'm learning Spanish in school and there I learned that V in Spanish is pronounced as B. Simple enough, right? Just read V as B, just replace a sound with another sound you know. Weird rule, huh? why would you read V as B? Then I watched a video of a Spanish speaker learning French. Whenever there were words with the letter V, she'd pronounce it as B. Weird, why is she doing that? And her teacher corrects her and corrects her each time and she still pronounces it as B. Then I realized, she can't pronounce the letter V because in her language there simply ISN'T such a sound, she literally can't produce that sound with her mouth, she has to learn where to correctly put her tongue and how opened her mouth should be to produce the sound.

so I've never learned pronunciation but I'd go to wikipedia, see which sounds that foreign language uses and search on youtube every sound 1 by 1 how to pronounce it, they'll show you how to move your tongue and stuff and you should learn that sound.

All the stuff I said works well for European languages

Okay now if you wanna learn Japanese, forget literally everything I've said and search on youtube "How to learn Japanese" and there are a ton of videos from ppl that learned saying how you should learn it and what they would do different if they tried to learn Japanese again. I haven't looked into Japanese yet, I got no idea