Start by writing what you enjoy reading.
It’s usually the easiest way.
You’ll need to learn, and how fast you can quit your day job depends on how fast you’re willing to learn, which in turn is a function of butt-in-chair time. The more you’re willing to work, the faster you’ll accomplish your goals. This is true in any field.
Translation: less dreaming, more writing.
Secondly, be prepared to test a lot of things, and abandoning them fast. Learning to write is a series of try-fail cycles until you figure out how your brain works so that you can develop a system of writing that works for you.
Corollary to the above: read lots of craft books. Try what they suggest, but be prepared to abandon it quickly if you feel that it doesn’t work for you.
Corollary to the corollary: you need to give each method a serious try. It’s not enough to read a book and spend 15 minutes “testing” the method. Try writing a complete story using it (short story if you work that way, but some people can only do novels – you’ll notice how your writing turns out.)
Formal training isn’t necessary. Practice and an overview of craft is.
Translation: read craft books, watch craft videos on youtube, listen to writing podcasts.
Once you got the basic aspects of craft down (shouldn’t take more than a few months/years, depending on how much time you’re willing to invest), start trying advanced methods. Don’t try to become an expert on any given level, be it grammar or the hero’s journey, before moving on. The key is to know enough to use it, not know it well enough to write craft books on it.
Translation: don’t get stuck in perfectionist mode.
Other than that, there are no real rules for how to become a writer. Well there are two:
BICHOK and WIBBOW
BICHOK – But In Chair, Hands On Keyboard. Translation: there is no better way of learning to write than by writing. Practice, practice, practice. Never give up, never surrender, stop doom-scrolling the news and watching 1970’s TV Sitcoms all night. Write. It’s the way.
WIBBOW – Would I Be Better Of Writing? Translation: you will get distracted. By craft books. By marketing books. By morons telling you to build your platform before you’ve even written your books. Don’t. Always ask yourself, would you get more out of this activity, or more out of writing and practicing? WIBBOW. It means Write.
That’s the only way.