You've got the seed of a story idea, possibly germinating it for some time, and you're ready to get that idea down in an organized manner. That little seed of an idea is going to grow to a flourishing PUBLISHED book, but there is quite a bit of work to be done.
Let's have a talk about organizing, and that HORRIBLE, DISGUSTING word I hate ever since middle school...outlining. Hated it then, hate it now. Good thing as authors we can take certain liberties with outlining to make it more useful to us and quite possibly, enjoyable. (Not holding my breath)
Your seed idea, or multiple linked ideas are still a little ball of creative energy, time, and diligence. It's time to unpack it a bit and see how the links behave.
For instance, I have an idea for a noir detective, the paranormal, and San Antonio. This is purposefully general to give you an idea of my process. Normally, I'd like my linking ideas to be a lot more specific, but we can run with this, because the process remains the same. Ask my kids, I'm big on processes.
Since I write with a movie camera in my head, my first thought is to setting and tone. A noir detective is a certain tone, but I'm thinking modern times, versus the 40's. What makes this character different from all other noir detectives....hmmmm...oh, I know. He's an angel. Better yet, a Fallen Angel. What's his name? Raphael, no, not that one. (Already have a running joke for the story)
How is this organizing? It's not, yet. It's still creative energy, but we're getting there soon.
So, noir detective done. He's a fallen angel, so paranormal is done. Set the story in my home town of San Antonio, TX. Three linked ideas, down.
For writing purposes, I use Scrivener, simply because I love their cork board. As I build chapters, I add note cards to the cork board. See? Told you organizing would be coming. If you don't have Scrivener, which is fine, any word processor document can serve the same purpose.
So, my fallen angel noir detective in San Antonio needs something to do...this is where the specific linking ideas I mentioned earlier would help, so let me do that. Fallen Angel detective must avert a war between the heavenly host and a hoard of demons. Twist ('cause there has to be one) is that the war is being pushed by devils with the demons as their pawns.
Index card number one on the cork board...Fallen Angel Detective Agency (Like that?) opens with Raph slinging playing cards into a hat, and missing a lot. Beautiful woman (demoness) comes in to hire him to check up on her boyfriend, she thinks he's cheating. Raph takes the case.
There you go, one index card and a general idea for a first chapter. For subsequent chapters, the process repeats. Using this method, you can work through the entire idea of your book and find other little things that fit in to flesh out your story. For instance, Fallen Angel Detective Agency has to deal with demon gangs in its investigations, as well as dealing with the Archangel Michael (yes, that one) popping up now and again and itching to fight the demons. Oh, and he doesn't like Raph very much.
Find a place in your loose outline for all the story arcs and you've built a pathway to writing your book. The things they taught in school were more suited for professional papers and such, so we authors can take artistic license and make the outline process work for us, rather than against us.
Happy outlining folks!