Anki (or a Leitner box) is the recommended method from the book "Fluent Forever"
After learning one language in school and a few others ad hoc on my own I gave this method a try.
I quickly gave up, though, once I discovered Dr Krashen's work on comprehensible input, and how boring and time consuming creating and studying Anki is.
Dr Krashen's work says that we learn language when we understand it. I.e. when we receive input at our level. There are quite a number of Youtube channels that claim to teach through this method, but many of them still resort to drills, themed vocabulary, grammar etc that are all pointless in the CI method.
The best one I've seen so far is Dreaming Spanish.
What's so cool about the CI method is that the spaced repetition is built in, implicit - if you're getting input at your level you will be getting repetition.
Children's shows are a great way to get input. Many countries have TV apps on iPad and can be viewed free with a VPN.
As an experiment I had my daughter only consume German media (living in Sweden) för a few years and she can now speak better German than I can, and I have never corrected her. She has broader and more modern vocabulary, naturally.
I find Anki essential for language learning. Did it for French, starting from 0 and finishing at B2 (except for writing) in two years (also taking a 3h / week course).
Especially at the very beginning, I find learning the first 500 words as described in "Fluent Forever" very helpful. Going through that before starting with the course makes the experience much easier.
I totally agree with Krashen's theories, but using Anki is a secret weapon here that puts the process on overdrive (e.g. reading on Kindle and then later going through words that I checked in the Kindle dictionary and adding them into Anki).
Also, it's essential to retain the knowledge, e.g. if it's a language that you don't use daily.
5 years and 30'000+ cards later, I find Anki indispensable in learning anything which requires memorisation. Thinks I have to learn for work, for hobbies, certificates, improving my vocabulary in languages I care about. Basically, every fact I care about strongly enough, goes into Anki. If not enough, goes into Obsidian.
Personally, as an intermediate Japanese learner, I have been careful to choose the right input just as Krashen advocates, but I also find Anki indispensable. I found to my surprise that I have been improving about equally quickly using Anki+Animebook+Yomichan for 1-2 hours a day while living the US as I did during an earlier period when I was living in Japan (but without access to computerized methods beyond a basic pocket e-dict).
As a beginner, appropriate input was enough to care of "spaced repetition" on its own, since children's media constantly rotates over the same small set of vocabulary. But after I improved past the ~2000 most common words or so, it happened more and more that a word I recently learned didn't appear again in my input until it had already been flushed. The probability that I would actually learn a new word for good progressively decreased as I picked the lower-hanging fruit, which is the cause of the dreaded "intermediate plateau".
I gather Anki pushes out the plateau much further: I have heard that it starts to feel Sisyphean to learn new words with it around the 15000-20000 word mark instead.
You have to learn the high frequency words/phrases in context (forget 500 most common vocabulary lists -- those don't work). As a child, you do this naturally.
However, there are certain situations where Anki helps. I find it's with words that are useful but don't occur frequently enough to pattern match. For instance, the word "ad hoc" in English -- it occurs in professional speech, but not quite often enough for you to remember what it means in context. This is where Ankifying can really help.
Ankifying commonly confused words/phrases can also help. For instance, in English the word "put" can be used in so many contexts, and many of those contexts don't occur frequently, but can be the source of funny mistakes.
"Put up (with)" and "put out" and "put in" all mean different things in different contexts. Embedding context in your Anki cards will help you recall these contexts.
You have used so much jargon in your comment, that as someone who is interested in learning languages but hasn't gone into the rabbit hole for years (90% of HN?), it was tough. Perhaps says what you preach about "receive input at our level".
After learning one language in school and a few others ad hoc on my own I gave this method a try.
I quickly gave up, though, once I discovered Dr Krashen's work on comprehensible input, and how boring and time consuming creating and studying Anki is.
Dr Krashen's work says that we learn language when we understand it. I.e. when we receive input at our level. There are quite a number of Youtube channels that claim to teach through this method, but many of them still resort to drills, themed vocabulary, grammar etc that are all pointless in the CI method.
The best one I've seen so far is Dreaming Spanish.
What's so cool about the CI method is that the spaced repetition is built in, implicit - if you're getting input at your level you will be getting repetition.
Children's shows are a great way to get input. Many countries have TV apps on iPad and can be viewed free with a VPN.
As an experiment I had my daughter only consume German media (living in Sweden) för a few years and she can now speak better German than I can, and I have never corrected her. She has broader and more modern vocabulary, naturally.