Okay it's not professional level, in fact my writing is a mess of things.
So, I talk to myself and I act out most of my books in my head. I'll actually tell the story to myself, and strangely I have ALL my books somehow memorized. Another thing is that, you all see I have like more than 20 books I'm working at once, but there is A LOT more books in my mind and drafts. Like ah!
So, usually for me a book can start from a song, a movie, a roleplay or another book. By this I mean the inspiration starts and it kinda just begins the story. I don't exactly plan it, it just happens, I get into "character" I should just I just tell myself dialogue.
So, A ton books become a mix of songs and movies and so I have playlists for my books basically.

And that is my process kinda. I warn you if you continue to read I may spoil somethings, but my world I've my books, is complicated, and confusing. I highly suggest you read my finished book "Glory Of Who We Were" because that book will be spoiled in this book, because it really shows off my universe for my books.