It's very simple: as much as possible, read, write, speak, communicate, watch movies, listen to radio or audiobooks in the language you want to know.
Personally, my great desire to know another language helped me. And only then I started looking for a solution to how to achieve this goal.
If you want to know the language fluently, then short courses even on the same YouTube (polyglot, for example) are suitable
But if you want to be able to competently express yourself in a foreign language, then it all starts with grammar. Yes, it is very boring and not so interesting, but I think that this is the basis for further language learning. Build vocabulary, do a bunch of exercises on the use of grammar rules and new words, watch short videos where you understand any expressions. In learning English, for example, I really like these publics: vk.com, vk.com
Speaking is also an important aspect of learning! Each city has a spoken language course where you can put all the theory into practice.
Practice and nothing more! Even if you studied the language from the first grade, you know all the grammar perfectly well and easily solve language proficiency tests, this is far from enough to freely communicate, understand and (even more so!) Think in this language. I say this from personal experience (I speak English fluently both orally and in writing). In order to begin to understand the speech of an English-speaking person, it took me about 2 months of life in the environment of the language (in the country where it is spoken). And I came there prepared for the TOEFL! It took me 4 months to start freely, without hesitation and long thoughts, to communicate in the language with the locals. And only when I began to fully understand those around me and express my thoughts to them, I began to think freely in the language! Therefore, the answer is simple: to be fluent in the language, you need to temporarily "place" yourself in the environment of the language. Now I'm learning Chinese, I moved to China for a while. If there is no way to move, then pen pals / skype will help you. Good luck!
Hello! From my own experience, the more practice, the better. In general, nothing new. I moved from Russia 4 years ago and entered the university in the English department (at my own peril and risk). There were quite big problems with English, even considering that I had taught it from the first grade of school. I arrived, I could not connect two words. I went to lectures, understood through the word. Then he began to read a lot in the language, communicate with classmates. Since there was only one native English speaker in the group, I asked him to correct me when I make mistakes in speech and writing. In a year and a half I felt quite free. To be honest, I don't know how much it helps watching TV shows in a foreign language, but it is often used. Try it, suddenly it works! Now I am independently studying 2 languages (Czech and French) of my own interest. I don't know how successful it will be without practice, but an attempt is not torture. Good luck!
Learn, learn and learn again! :) Well, if we discard the standard methods, then probably the most effective (for me) method is to watch films and TV shows in a foreign language. First with subtitles in the original language, then without.