No good plot survives an encounter with your characters.
I suppose the title really should be "how I write a novel" but, I think I'll probably ramble on a little about various methods anyway.
Agonize endlessly about the entire series, all the interconnected plot lines, schemes, plans, and scenes that I want to see throughout the entire series, and see how everything culminates into this one book.
Call my editor, ask him a bunch of questions, further clarify the plot and have a really great outline.
Write approximately three paragraphs, and discard the entire thing in a sudden surge of inspiration
Realize that some things still have to happen in order for the next 30 books to make sense, and back-peddle, justify, rationalize, and otherwise make shit up.
Edit as necessary until things make sense
Write a few more paragraphs, and repeat steps 2 & 3 repeatedly.
Realize that the cool ending I had planned won't happen until "the next book".
(Case in point, the ending of "book 1" has yet to happen and we're in book 3...)
In all seriousness though, some people really struggle with writing a book. I think that almost always comes down to fear. Fear of judgement, fear of imperfection etc.
To this, I say two things.
1) Write a shitty book. Seriously, just do it. You'll learn a lot in the process and you can always throw it out if you don't like it, or hell, maybe you'll be able to edit it into a good book. Either way, you'll be ten times the writer after completing your first book than you will after 10 years of thinking about writing it.
2) Write for someone. Write for an audience who loves your work and is very easy to please. EDIT for someone who is impossible to please... once the book is finished and you want to tear it to pieces, THAT is when your perfectionist friend comes to mind. But writing... writing should be done for that superfan who loves everything you do (regardless of how many commas you use ;)
Beta readers. Get ones from all walks of life. Ones who you think will love the book, ones who you think will hate it. Ones you think won't provide any useful feedback, and ones you think will. Mix and match, stagger the feedback, and weigh whose you want to listen to.
If your experience is anything like mine, you'll be shocked at whose feedback ends up making the book a masterpiece and whose feedback you'll end up disregarding.
My target audience for book 1 was males aged 16-26. , grab a few old farts, some moms, some people who
Learn a lot. Podcasts are great.
Write another book. Either the book you wrote was good, or it wasn't. But writing another book is always the next step.