Learning languages is kinda my life-long hobby, and I'm quite stupidly attracted to the hardest languages like Chinese, Japanese, Arabic, etc. To have any chance of achieving my goals I delved deep into modern techniques and found quite a few that didn't work, and some that did.

Immersion is key. This doesn't mean you need to go live where your target language is spoken, although that certainly helps. There are expats living abroad who never learn more than 3 words of the local lingo, and there are dedicated polyglots who never leave their dorm room and yet master some of the hardest languages. You have to create the immersion environment, and thanks to modern media (streaming TV or radio, movies, podcasts, audiobooks, the Internet) this is something you can do anywhere. Put stuff in your target language on all the time, in the background while you are doing other stuff. First it is to get used to the sounds, then as you progress in your studies you'll start paying attention to individual words, and then finally meaning.

As for your studies. You will proceed in 3 phases: Beginner, Intermediate, and Advanced, and each phase what you do will be different.

At the beginning, your job is to get an intuitive feel for the sound of the language, learn basic grammar, and memorize enough words to understand the gist of everyday, non-technical language. Get a decently good textbook for your target language and work through it. Memorize the meaning and use of every word given in that text (usually around a thousand or so) and practice all the exercises. Use a flashcard program to drill and maintain your knowledge.

At the intermediate level--which is beyond what most students get to at the end of the full language sequence at an American university--you focus on building up your vocabulary, advanced grammar, and conversational fluency. Work through some intermediate textbooks, but also start branching into graded reading. See if you can find "easy" material meant for learners. Often public television in the country that speaks the language you are learning will have "easy" programs meant for language learners. E.g. NHK has a daily Japanese bulletin that is in simple Japanese and has phonetic annotation for the Japanese characters. Start reading a translated book that you already know well in your mother tongue. Lots of people choose Harry Potter because it is (1) simple kids book, and (2) published in every language under the sun. My personal go-to is The Hobbit. If you can get it in audiobook and follow along, all the better. At first you will not understand much. Just circle the things you don't get and move on. You'll be surprised when you look back and most of the things you circled you will grok. But anyway, keep consuming media and studying intermediate sources until you get to the point where you only have a one or two things circled per paragraph. At that point you can start looking up everything you don't get from context. Along this whole stage you start collecting sentences. Every time you encounter something new that you didn't know and really want to learn, copy down the sentence that demonstrates it to your flashcard application. Review them regularly with an SRS program. When you get to 2000 - 8000 sentences (depending on the difficulty of the language relative to ones you know), you will be approaching Advanced level.

I'm only at the intermediate-advanced level or lower for my target languages, so I don't feel qualified to advise on that. But I expect to transition away from books or source material that I know well and into reading non-translated novels written by native speakers, and technical works in domains I am interested in. Starting in the intermediate stage and continuing into the advanced you need to have frequent conversations in your target language, which will help build up your spoken fluency.

If you happen to be learning Japanese, you will have an abundance of resources to use. Start with these:

https://www.wanikani.com
https://www.bunpro.jp
2001.Kanji.Odessey sentence pack (google it)
iKnow Core sentence pack

For Mandarin Chinese there aren't as many good resources. You're almost better off learning Japanese to an intermediate level (finishing Wanikani) then using Heisig's Remembering the Hanzi to learn the differences. Or at least that's what I did.

(There's a business project for someone who puts in the effort to clone wanikani and bunpro but for Chinese. Everyone: contact me if you're interested.)

For Chinese sentences there is a great deck of full-sentence flashcards:

https://promagma.gumroad.com/l/IEmpwF

The other languages I studied were European, and I mined sentences from old 1970's Assail courses, which were jam-packed with info. Their needs were not quite as intensive as Japanese and Mandarin.