The most important thing is to just start and stick with it, no matter how meaningless or small the thing you are doing is. You will jump onto something better as soon as you find it. However, if you haven't even started then you will always look for something that's even better than the best thing you already know about. i.e. you will do nothing even though your methodology or the instructional material you found is already more than good enough.
It's not so much that there's a one size fits all learning method, but rather that there are certain constants in language learning. Namely one must actually hear and speak the language.
Learning the grammar and syntax of the target language entirely in ones native language may provide some value for some people, but in and of itself doesn't teach anyone a language and is entirely optional. In general formal study of grammar is an advanced topic.
Not because it's necessarily incorrect but because it makes a huge assumption that all people learn in the same way.
I am fluent in 3 languages, have a decent grasp of another 2 (although cannot really speak much of either for lack of practice), and have picked up basic grasp of a couple others.
Most things I learn through extensive practice using theory on top to help me understand my practical results. Programming, engineering, robotics, electronics, physics, etc. Even much of mathematics!
With languages, the only way I start to truly grasp them is by analysing grammar, syntax and etymology, then starting to match those to pronounciation, and then I can go from there.
The idea that everybody should learn a language in way you deem better has hindered my ability to learn many, and has slowed me down a lot. I wonder how many other people learn like me and are currently out of options when it comes to language courses because of this kind of one-size-fits-all approach to teaching.