Personally, I can attest to the effectiveness of the advices of this video. That’s almost how I acquired Italian.
One thing I tell people when they ask me about language acquisition is that “language is a muscle.” That’s correct, IMO, on both a metaphorical and a physical level. For a muscle to get stronger, you need to (1) make it work and (2) shock it.
Physically, language is a muscle because its effect is produced through muscles: brain, tongue (spoken language), body (both spoken language [gestures] and sign language). Each language brings around new sounds (even when sounds are very similar and none totally new, subtle differences will exist) and new ways to organise sounds, and new suprasegmental features (use of tone, accentuation, etc.). Producing them will be a stretch on your tongue, especially (assuming spoken language). In order to get them down, you’ll need to do repetitive excercises, not unlike sports training. And being able to produce these sounds will enhance your ability to pick them up when listening.
Metaphorically, language is a muscle, because it develops similar to how a muscle develops: as I said, you’ll need to stretch and shock it. But in order to do both, you need to know which muscle you want to work on, and what are your limitations. In language, the stretch and shock comes from making yourself subject to stuff you don’t know well. But just like you can’t lift 50kg or 100kg if you never did weightlifting and did not develop your strength and you don’t know how exactly to use a barbell, you can’t lift it and may hurt yourself. So you need to study certain aspects of the language, explore certain patterns like e.g. where do the loanwords come from or what sort of basic grammatical structure is common (e.g. what’s the common word order, is it heavily inflected or not, does accent and tone play any roles in the language, what’s the structure of a word?). Study the most basic aspects of grammar (e.g. are there articles, what are the pronouns, how does the simplest most common tense work?). Go through the Wikipedia page for the language. Try to find out which sounds are new to you in the language. Think of this like preparing a good training routine, a good diet and gathering your tools and supplies for it. Once you put some time into this, it’s time to start seriously studying with input (tho it does not hurt to get your feet wet in the meantime).
While studying with input, repetitiveness and repetition are key. Repetition is doing something multiple times. It’s not watching the same video or listening to the same podcast many times. It’s about repeating a certain structure, a certain routine. E.g. “I’ll watch one recipe video every day” or “I’ll read one news article every day”, but not “I’ll watch this particular eggnog recipe by Jamie Oliver every day for a week” or “I’ll read this particular news article ten times until I understand it”. That latter sort of repetition might even be bad because (1) it’ll be boring and discourage you and (2) you may develop a “blindness” for certain aspects of it. So you should pick some things (for Italian this was cooking videos and news articles, and videos from a couple youtube channels), and repeat them often, but not too often (for me, it was watching a video and reading a news article almost every day, and when I looked at a Wikipedia article, I’d try to read the Italian version too, even if just a little bit of it). These things you pick need to be repetitive. Repetitive means that these things have a certain vocabulary and a certain structure. A recipe video is a very good example that I like to use often, because it has a certain vocabulary, a certain inventory of objects (foodstuffs, culinary tools, certain type of people), and a cetain set of actions (putting things into other things, mixing, washing, peeling, frying, etc.), and thus are a perfect source of “comprehensible input”. News articles are often similar, especially international news: they are relevant, and refer to events you probably know about, using a certain vocabulary. Even if you don’t literally comprehend the phrase “Asia Argento ha accusato Harvey Weinstein di violentarla carnalmente”, you’d probably understand what it’s about. When you expose yourself to this sort of content, that you somehow understand in whole, but can’t really decrypt the parts of, you’ll begin to subconsciously learn the individual words as you expose yourself more and more. Because they are repetitive: persons, countries, institutions, actions, topics etc. reoccur all the time.
One important thing to understand it that grammar (as in the formal documentation of a language), however it’s made, is a study of a language, and is not essential to learning it. People learned languages when grammars weren’t a thing, before Panini, before Musaeion, possibly even before writing, and, speculatingly, since as early as language became, in whatever fashion, a thing. This is not unlike how we don’t need to know physics in order to lift an object, or be a sexologist in order to reproduce. Language is something we do, we’re wired to do, and grammar is a study of how we do it. Using it as an adult may enhance the learning process, or ease it in the beginning, but the knowledge of a language is much much more than that.
It’s useful to conceptualise knowledge as a network of of data points. In order for you to reach a given data point in such a network, you need at least one node connected to it, and getting to it is going to be easier depending on (a) how close it is to your starting point, and (b) how much connections you have to such data point. If we assume, then, that individual words are individual datapoints, it becomes clear why dictionaries can’t help too much with learning a large number of words. You need to learn words in context of other words that are relevant to it (not etymologically or grammaticaly related, but semantically or syntactically relevant to it). When you read some text, each word is in that sort of context. Sometimes you cannot get any sense of the word, regardless of the context, or the ambiguity not knowing it impedes understanding a key part of the text. That’s the correct moment to look the word up in a monolingual dictionary. If you can’t understand the monolingual description, only then look up a translation. Apart from that, just ignore words you don’t know, but try to guess their meanings when you can’t.
One thing that’s very useful if you can is to try to “think” in the target language if you can (not everybody has an “internal monologue”). Initially, because you don’t have much words you know, you won’t be able to think about much. But try nevertheless. It helps with internalising the language. Just like how a pure software virtual machine is bound to be slow because it executes the instructions as it receives them instead of piping them directly to the hardware when it can, one is bound to be very slow at processing language when they’re mentally translating all the time. We need to be able to skip that step, and thinking and talking to yourself in your target language helps a lot with that. It also helps build the network I was talking about. If you can skip the translation step, it means you can parse and produce real-time language input, and that’s what enabled true fluency and true comprehension.
Also try to produce: if you have native speakers around, you’re incredibly lucky, talk to them, it’ll make you so much faster. In any case, write in your target language. Write a diary, write some of your notes in it, go to your language’s subreddit and write there, or better, just go to the country’s subreddit and participate. Don’t be afraid to make errors, and tell trolls that make fun of you to go get a life. That’ll help you develop the productive “muscle”, and also expose you to slang of and to the culture around the language you’re studying (except English, because with it you’ll still get these, but then a whole lot more).
Lastly, note that each of the four basic linguistic competences (speaking, listening, reading, writing) have their own “muscles” which need to be trained independently: being good at one of them will help with others, but you still will need to independently work on them. Continuing the training analogy, walking and running may help with your butt’s shape, but if you want it to look good, you’ll still need to do the squats. You can throw strong punches if your arms are strong, but in order to be able to punch reliably and effectively, you need to work on punching. An actual trainer helps a lot, but a punchbag is way better than nothing.
The above paragraphs summarise how I learned first English and then Italian to very competent degrees, in all four of the four basic linguistic competences. I devised my method from reading up on different, especially newer language learning methods while preparing to learn Italian. I knew English very good at that point, having had a lot of practice reading and watching a lot of material from learning programming (I already knew basics before, but I could not speak fluently or read or understand much authentic material). I combined what I learned from reading on language learning with my experience I had with English, and stuck to the method. Thus, I advanced quickly, being able to read complex, natural texts in a few months, comprehending a very large portion of them. In less than a year, I was able to appreciate proper literature in Italian, and write rather complex texts without prior preparation. And this with fifteen minutes to an hour or so studying per day (not very strictly every day tho), depending on how busy (or lazy) I was in a particular day.