From experience, you gotta learn that language is like a filter through which ones' mind/heart/intentions are shaped. Then, you master getting into the pre-language state of mind that has no language, and from there you train yourself to see the world with a different organization of concepts.
First, learn the 200 most common words in the language. Do whatever it takes to learn all the verb conjugations. Spend countless hours listening to real humans talk in conversation and repeat exactly as well as you can over the tracks whilst listening (like singing along to a song) so you can master the ups and downs of tonation in pronunciation.
Top 2 tips: One) 15 minutes every day is worth so much more than 150 minutes in one day every 10 days. Meaning: frequency is key, and quality over quantity.
Two) to truly become "fluent," that is, to make the leap from "this is something I can use" to "re-experiencing life through the frame of a German person / Japanese person / Swedish person / whatever" you must spend time in a place culturally saturated in the language.
That said, you can spend 2-3 years of your life immersing yourself in radio, news, television, movies, and learning mnemonic methods (whacky stories and images that help you retain meanings) and make incredible progress.
I'd also suggest learning sets of words, as you say the most common. I started with days of the week, months of the year, numbers up to 20, and similar.
Though I'd probably suggest having lessons, be they official at a college/university/academic location, or low-key chats with locals over coffee/beer/cake is a good thing to do. I had a lot of questions about grammar and how to chain words that were easily resolved with chat.
That said, if one can't do the lessons, I fully suggest working through some of the adult learner workbooks - the ones for immigrants. Some are more grammar-heavy than others. The draw is twofold: You learn some common speaking terms plus you get introduced to grammar, complete with exercises.
But outside of that, there are some great suggestions. A few things I have to suggest:
Get ahold of some of the books and workbooks for adult learners. This is an easy way to learn grammar and common phrases. I'd use these in conjunction with other things.
Next, watch children's cartoons. Here (Norway), adult films often simply have subtitles and leave the English audio, but children's shows are dubbed with simple words and clear language. It is a bonus if it is a crossover - I watched many hours of "Spiderman" cartoons in Norwegian.
Read books, especialy ones you've read in English. Harry Potter, for example, is available in multiple languages. It'll be difficult at first, but you'll increase vocabulary from the repetition in the book. Terry Pratchett is another choice for German, at least (I can find some in Norwegian).
For speaking, Skype is popular for tutoring and speaking practice, though you might have to pay for this.
Good luck :)
If you're not in Germany or Russia, get a teacher with a small class. Mine was 4 people so we could speak and exercise a lot.
The first book I read was Beauty and the beast in french. It's a short story to start with. It took me a long time to just read some sentences with proper pronunciation and I'm still working on it. I look up youtube videos for some extra info. The frenchpod101 videos are good. I guess you might have already tried these, but just wanted to mention it anyways. Hope this helps! Good Luck!
I've been doing Duolingo's French lessons every day for 15-20 minutes during my commute to work. I find it quite engaging and useful.
A lot of language learning exists below the "level" of words and sentences. The sound of a language.
And even when you progress to words and sentences, phrases, idioms, and the like differ. Gender exists, or differs. Native speakers don't memorize these. They learn by absorption. You are fortunate in that, with today's connected world, so can you.
P.S. The rest of it, e.g. working through texts, tutoring videos, and all that, I leave to you. Just remember: EXPOSURE. And that you can't and shouldn't try to consciously process and monitor it all. Steep yourself in it, and let your whole mind (including sub-conscious, or whatever we're calling it) process it.
P.P.S. Approach it this way, and you will find that some of the "much harder than when a child" belief doesn't actually really of fully apply. I became conversational in French, starting from zero, in about seven weeks when I was 22. In the middle of Vermont.
I also use online flashcards (Anki can be a useful tool for making your own online flashcard decks).