Here are a bunch of things I tried on my journey towards learning Japanese and some advice on what worked and what didn't. I started learning Japanese seriously in April 2012, and passed the JLPT N2 test a few years later. Everyone learns differently, so my advice is unlikely to be universally applicable, but I will try to lay things out in a way that will help you make decisions about how to go about your own language learning journey.
Identify the primary reason why you want to learn Japanese. Japanese is very different from English, and it can be daunting to try and tackle all aspects of it at once, so anything that helps you prioritize where to put your time and effort to accomplish that goal can be helpful.
If you have no intention of ever writing in Japanese with pen and paper, you should slice up your time to emphasize other aspects, and not worry too much about memorizing things like kanji stroke order or practicing your handwriting regularly. My Japanese handwriting is horrible, and I am embarrassed whenever I need to sign my name in katakana at the bottom of a receipt or something, but it only comes up once or twice a year at most, so I think that's acceptable for my set of priorities.
If you are primarily interested in playing video games in Japanese, you will probably want to focus your efforts on reading comprehension and listening comprehension over any kind of composition skill or writing ability. If you are interested in conversing with native Japanese speakers while travelling, you will probably want to focus on listening comprehension, formulating sentences (and once you've got the basics down, doing so under time pressure), and pronunciation.
This all seems really dumb and obvious, but a surprising amount of people don't do this and find themselves overwhelmed by trying to give every aspect of the language learning process equal time and priority. Prioritize what you need most and you are likelier to stick with the learning process, because you'll progress more quickly than if you try to do everything at once.
Traditional textbook learning is honestly overrated.
The Genki series is often recommended and used in college Japanese classes. It's a decent way to get a quick start reading and writing the hiragana and katakana phonetic alphabets, as well as getting a general feel for how sentence structure differs from the English language. The pacing slows down significantly in the second half of the first textbook, and you could get a more complete explanation in less time by consulting some of the free resources available online, such as Tae Kim's Guide to Learning Japanese. I've poked around some alternative textbook recommendations, like Japanese for Busy People and ultimately, most of them are fairly similar.
Beginners to the Japanese language often fall into this trap where they think everything would be much simpler if kanji (Chinese characters which refer to concepts) didn't exist and everything was written out in hiragana or katakana (Japanese's two phonetic alphabets). This is an opinion you will quickly reverse your stance on once you get more proficient.
Kanji are incredibly helpful in disambiguating what a word means, because lots of words share the same phonetic reading. Knowing the readings of any kanji on its own isn't particularly useful. If you spot a word for the first time, nothing will tell you which reading is being used, so in my opinion, it's more helpful if you can learn what the kanji mean in a large sense, so that your first encounter with a new word can be an educated guess of what that word means. You can worry about learning the correct reading of the full word, and not the kanji, later.
A classic example would be the word 消火器. At the end of the day, is it more useful to recognize these characters as fire extinguisher because the three kanji vaguely mean disappear, fire, and tool respectively, or to know that it is read shoukaki? You will eventually learn both, but learning kanji meanings will at the very least let you get a decent idea of what kind of thing is being discussed.
Remembering the Kanji Vol. 1 is a book that tries to give you helpful mnemonics to associate meanings to the most used (2000+) kanji in line with this kind of thinking. Volumes 2 and 3 focus on giving you mnemonics to memorize the readings of various kanji, but I don't think kanji readings are that useful in isolation.
You will find a lot of mixed opinions on Japanese learning message boards about this book. I think it is immensely useful, but it is a slow burn. You will only notice the impact it has on your ability to understanding things until much later in the learning process, and I think a lot of the negative opinions I have seen about it were from people who gave up on the learning process too early to appreciate it.
A practical example of this would be from playing JRPGs. A lot of them feature words from domains you may be less familiar with (like aerospace or the military) or made-up terminology, whether it be organizations, names of supernatural phenomena, or sci-fi technologies, and being able to piece together a vague sense of what that unknown phrase means is much better than being able to read it phonetically and still have no idea what it ultimately means. If you aren't actively consuming media intended for a native Japanese audience and are sticking to the safe confines of beginner-focused textbooks and resources, you are definitely not going to think studying RTK Vol. 1 paid off.
If you think you are in this for the long haul and you are prioritizing reading comprehension, I do recommend drilling these meanings into your head, and starting fairly early, as there are no real prerequisites for this book to make any sense: it's all mnemonics. I used koohii, a RTK-focused kanji spaced repetition system website alongside the content from the book to study these.
What's a spaced repetition system?
Spaced repetition software such as Anki is an automated way to drill various kinds of flash cards regularly. When revealing the answer to the flash card, you must evaluate if you got it wrong, or if you got it correct but judged it to be hard/normal/easy to remember. If you get the flash card wrong, you will see it repeatedly until you get it correct throughout your current study session. If you get the flash card correctly, how easily you remembered it will determine the interval until the next time it will be up for review. This is a proven technique that increases your learning rate.
koohii as mentioned above is an SRS system built specifically for people studying the RTK books, but Anki was the go-to app on for desktop and mobile to do this with publicly available stacks of virtual flash cards that you can download via their directory.
At the time, I used the iKnow Core 1000/2000/4000 stacks of flash cards, and I would study every night for an hour. These flash cards contained words, their English meanings, Japanese sentences, their English translations, and audio files of all the words and sentences. You were tested on English to Japanese, Japanese to English, for both words and sentences, in both text and audio. The 1000/2000/4000 number in the name of the stack refers to the vocabulary words covered in the flash cards being the top 1000/2000/4000 words used in the Japanese language. You don't need to learn that many words to get pretty far.
I believe iKnow's flash cards have since been locked behind a paywall, but the pricing seems pretty reasonable for what you get. I believe they now use their own app for these instead of Anki, but I have never used it and don't know if it's any good.
If you are decent at pattern recognition, you can choose to go off the deep end like me and rely entirely on the flash card deck for grammar learning as well. After having completed that first half of the Genki workbook, I solely picked up knowledge of Japanese grammar from recognizing patterns in the Core 2000 flash cards I was studying at the time, only resorting to looking things up in grammar resources if something was hazy. An hour of flash cards every single night is the only formal Japanese study I did after learning the RTK meanings and completing the first half of the Genki workbook, up until I took my JLPT N2 test. I think I'm personally predisposed to learning this way, because it's also how I pick up programming languages by jumping right into reading some source code.
Anki also lets you create your own stacks of flash cards. You can use this to study words or sentences pertaining to your specific interests that may be outside the scope of the Core vocabulary stacks. Every once in a while when I had time on the weekends, I would go to online publications about tech or gaming and mine them for vocabulary words I didn't know, only to put them in my personal flash card stack. That way, I could grow my vocabulary in areas I cared a lot about, and then use those words to increase my comprehension of publications in those spaces and make it easier to communicate about those topics in Japanese.
I started incorporating media targeting a native Japanese audience into my media diet fairly early in my learning process.
I was a heavy anime watcher before starting to study Japanese, so I just turned subtitles off and carried on as usual, watching the same kinds of shows I was watching prior. For the first year or so, I didn't really have trouble understanding the gist of what was going on thanks to the visual information, but I wouldn't pick up on details in the plot because my Japanese was too poor. But overall, there was only one show where I didn't really understand anything that was going on, and that was April 2012's Hyouka, a mystery slice of life anime about a classical literature club. It leaned into literary references a bit too much for my skill level at the time, and to be fair, it was one of the first five shows I watched entirely in Japanese.
There's some debate as to whether immersion time through media consumption should be done passively (by just watching the show as you would something in English) or actively (mining shows for vocabulary and sentences for use in flash cards). I tend to lean on the passive side of this argument. I was just substituting time that I had mentally budgeted for entertainment with entertainment in another language, so I didn't want it to end up feeling like work. Ultimately, what was most valuable to me was passively getting accustomed to the sound and rhythm of spoken Japanese, and getting weekly validation that my study was paying off as my comprehension got better over time, specifically in media intended for a native Japanese audience, because that was the media that made me curious about learning the language in the first place.
A few months later, I felt comfortable enough in my basic reading ability to do the same for video games. Almost every game I've played on a system other than the PS4 since summer of 2012 I've played in Japanese, including fairly long and text-heavy ones like Persona 4 Golden. Video game immersion will inherently be more "active", because things explained in character dialogue may be crucial to your ability to progress in certain games. You'll need to look things up a lot more, and you won't necessarily be able to rely on visual information to carry your understanding of a situation like you often can with TV and movies, especially on older systems.