* There's no silver bullet. If one 'method' was far quicker and easier than all the others then everyone would do it.
* Generally it's a long process. You need to find a way to study and learn that you can do for multiple years.
* A teacher's explanation won't make things that much easier. When looking for a teacher, find one that is fun, exciting and motivating for you.
* Avoid translation as much as possible. The language you are learning will have lots of differences to the language you speak already. Don't get hung up on them.
I think adastra22's comment goes into some good detail about a method to learn a language. Try to have the attitude that you are a detective, investigating what all these different words and phrases are and what they're used for.
Definitely focus on phrases and sentences, not words. Sentences contain a lot more grammar and context.
By using it you have to rely on your memory to understand it and speak it. Start with simple conversations and move forward.
Reading books and watching videos will give you a false sense of knowing the language. There are people that can pass a high level language test but would not be able to have a conversation. So, speak it ASAP and everyday
Duolingo is not the way to do it. It puts playing over learning. Ultimately there are faster ways to learn than the app.
Personally I found when learning things like a new language, having a real tangible beneficial goal in front of me is more important than which method that I use. For example being in a relationship with someone who speaks the language that you want to learn and being cut off from your native language provides a tremendous tangible immediate goal to quickly pick up the language.
I kind of equate this to people who Windows only users and they ask me how to learn Linux. They'll indicate that they've taken some courses online and bought some books and read them and so they know these basic things about it but I tell them if they really want to learn it get rid of Windows and only use Linux. Because at that point you force yourself into a situation where if you want to do something there is now only one way to do it and it's not the way you previously knew so you must learn. That also causes you to learn in a more natural process where curiosity and problem solving become the things at hand rather than a consulting a book of rules and syntax.
When you learned your initial language you weren't taught it from rules and syntax out of a book you were simply immersed into it and you had a tremendous impetus to learn it.
If you can't move there, live with someone prob
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.