Taking that day off to deal with repairs must have worked because one of those "but WHY are they doing this?" issues I'd been struggling with suddenly resolved itself the second I sat down to work yesterday. The subconscious must have been working while I was otherwise occupied.
Since I've been brainstorming a book, I thought that would make a good writing post topic. How do you get started on a book once you have an idea? Here are some things you can do to get started. You don't necessarily have to do all of these things this way and in this order, but maybe this will spark some ideas of things you might want to try.
I usually start with a brain dump, writing down everything I know about the idea. I get most of my ideas while I'm working on something else that I need to finish, and this works as a way to clear my head so I can get back to that project I need to finish. I keep a loose-leaf idea binder, and when a new idea strikes me, I write down everything I have in my head, then put that sheet in the binder. If a new idea strikes me, I add it to the page. I often find that the idea multiplies while I do this, so that even if I start with one sentence worth of idea, I'll have at least a page when I'm done. This doesn't have to be in any kind of logical order and doesn't have to make any sense. It doesn't even have to be in complete sentences. Just scribble down whatever comes to you -- characters, plot events, scenes, settings, imagery, lines of dialogue, clothing, scents, etc.
If you're a visually oriented person, you might want to do this in mind map form. That's when you start with your initial thought in the center of the page and then draw branching spokes of related ideas radiating out from it.
If you've done the brain dump and put it aside while working on something else, when you come back to actually develop the idea into a story, review all your previous brain dump notes and write down absolutely everything else you can think of, including things you want to happen, even if you don't know how they'll happen. It will probably snowball, with ideas triggering more ideas.
At this point, I usually make a list of things I need to research. Not nit-picky, specific things, like the size of carriage wheels in 1836, but locations, career fields, related history, sociology, psychology, mythology, clothing, etc. This research is more for idea generation than for knowing specific facts. Some authors try to avoid reading novels that might be similar, but I like to at least get a sampling so I'll know what the tropes are. I like to find some of the classics of the subgenre, some of the more recent (like within the past decade) bestsellers and some of the newest books by newer authors. I tend to mix genres, so that requires reading books from both the genres I'm mixing. If I'm writing something in a historical setting, I'll read books written at that time to get a sense of the language and cultural mindset. I take notes as I read, not so much for facts (though I do keep a list of books that will be helpful when it comes time to get facts) but for ideas the books spark or concepts in the books that I think might be relevant. For a fun break in all this research, I may watch movies or TV shows that remind me in some way of the idea -- the setting, the cast, the themes, the kind of story. If my idea involves a particular culture, I may look up recipes and cook something like my characters might be eating.
After all this input, I'll do another brain dump and write down everything that's come to me. I'll review all my notes from my research and from the brain dumping, and then I'll start trying to mold it all into a novel. I'll take a story structure like the hero's journey and brainstorm what might happen at each stage. I'm not really making an outline here, just listing things that could happen at each point. Then I'll go back and see which ones I like best and look at how they could all flow together. I've written books where the plot just came to me, all at once, with almost no brainstorming required, but most of them require a bit of teasing the plot out of all those ideas. Sometimes the characters are more real to me than the plot, so I may start there and come up with what their story arcs are before I try to create a plot. Basically, I start with whatever's the clearest and then solidify it from there.
People who don't like to plot may just start writing at this point rather than doing that outlining work. That thought terrifies me, so I can't offer a lot of advice for writing that way. I have heard of "pantser" writers who, instead of outlining, make a collage, where they collect images or items that remind them of their story and put them together in a way that makes some kind of sense. I usually do a musical collage in the form of a "soundtrack" for the book, with music that reminds me in some way of the characters, story, scenes or emotions. I don't listen to this while I'm actually writing, but I may listen to it as I do other things, or I may listen to the relevant parts before writing -- say, if I associate a song with a scene or with an emotion I want to convey in a scene, I'll listen to that part of the soundtrack before I sit down to write that scene.
The big question is, when do you start actually writing? I like to hold off until I'm impatient and eager, when all this pre-writing work is just a delaying tactic and when I can see the opening scene in my head as clearly as if it were being projected in a movie theater. Then I know I'm ready. If I force myself to start, then generally that means the story isn't yet ready for prime-time.