Leer libros. Leer, leer y lea de nuevo. (Read books. Read, read and read again.)
This is actually good advice. I've read a shit ton on language acquisition and how it happens, and believe in the Krashen method; massive amounts of input and focus on reading and listening over speaking or writing.
The best way is to find things you enjoy, and read about THAT or watch videos that are interesting to you. Keeping your focus on interesting content will drive you to continue learning. Prepackaged courses are garbage and produce poor results. Anki is good for starting out and aquiring vocabulary.
I started with a cracked version of Rosetta Stone when I first began learning languages. I enjoyed the learning process itself, and becoming accustomed to letting my brain organically pick up on clues as they came. The pictures provide contextual clues, so it helps you solve the language riddles (so to speak) more organically instead of just giving you a phrase in English and then one in Spanish.
Once you can read in your target language, you will hit an amazing growth spurt in learning. Challenge yourself with short stories and books with upwards to 50% unknown words, and this part for me was key:
ENJOY IT AND GET CURIOUS/INTERESTED IN THE STORY!
Then, read it again and again until you know 80%-90% of the words. You will actually find that although you don't remember all the words and what they mean, you'll find yourself getting the gist of things and following along with the story.
In the last stages, you want to focus on speaking but ONLY after doing tons of reading and listening, otherwise you risk fossilizing a really shitty accent. Learn naturally like children do. Children absorb tons of information like a sponge and only then do they begin speaking. Adults have the advantage over children in that they can read, further accelerating the language learning process.
Just be careful not to speak too early. Way too many "experts" and "gurus" will try to get you to speak right away. Those "SPEAK XXXXXX in 7/30/90 DAYS!" courses are the get rich quick scheme of language learning. It's the equivalent of the instant millionaire dropshipping course or SEO course or what have you. Just because you can orally shit out accented phrases, doesn't equate to knowing the language.
Just like any discipline, it takes time, patience, and dedication to become fluent in another language. I suggest speaking after you can read and listen to most natives without having to ask them to repeat themselves. From there, you can often do language exchanges with foreigners to practice your speaking skills. Same for writing -- join forums in the target language that revolve around topics that interest you. If you like politics, for example, join political forums and social networks (FB Groups, IG influencers, etc) in your target language.
TL;DR; Keep it interesting, be patient, and focus on input over output.