I come at this idea from a different perspective than many. I do not suggest here that my method is right when all others are wrong. "Whatever gets you there" John Lennon said and he was right. This is simply "my" method though I am sure that many other authors have written a book in the same way as I am now going to suggest.
My first, second and third novels were traditionally published. It was easy for me. This does not mean that you cannot learn the trade, many do and coming from nothing write something utterly compelling. Often better than my, unusual turgid fare. Rather it is mostly a case of consideration and luck. A well considered tale, well thought out, with adequate writing will carry you farther than glossy writing that lacks substance.
My first "true" suggestion for aspiring writers is; pick one story. Only one (for you will have hundreds in your mind). Type it out. Have two glasses of wine. No more, no less and think on that tale to the exclusion of all others. Think about it for weeks to the exclusion all you have. Imagine how it will play out. Then write an additional tale based on one of the characters in your original, then another based on another character, then a tale that combines them and keep going. Lol- you will have months of editing, creating a consensus between those disparate tales but months later, perhaps years, you will have a book that others wish to read.
I rarely give advice on writing as I seem to come from a place where few others do but this is my little bit of guidance.