My first book idea started on a smoke break, was just shooting the shit about time travel books we would love to read. After that I wrote a short story and felt pretty good about it so I sent it to my coworker, they asked for more so I wrote more.
It's crystalized gradually, over months, really. Just coming up with specific scenarios, characters, themes, as well as other various tidbits, writing them down when they come to me, and constantly thinking "Okay, what's the through-line here?" Over time this builds out a world where, once I feel it is established, I can then constrain myself within to flesh out a coherent, limited set of major ideas and hit the ground running with it.
I guess, personally, my major piece of advice is that you probably will not be able to write everything you want to write within the same story without it losing a sense of coherency as a creative work. You're either lucky or plain if you can manage to encompass the entirety of your ambitions into one linear plot. Start chunking your preferences into groups. Develop an internal set theory for your plot bunnies. Figure out what works well together and what doesn't.
And as always, the most important piece of writing advice is that you actually have to start putting pen to paper/fingers to keys rather than infinitely graphing out your potential story. Your internal universe may be pristine but it matters naught if others can’t indulge in it. It’s like programming, premature optimization is a path to never making anything, the most important thing is that it runs and functions. Tinkering is a purely contextual activity. What use is designing the perfect jet engine if, once you’ve realized the parameters, you find yourself deciding that an elevator is a more appropriate mechanism to arrive at your destination?
I guess, personally, my major piece of advice is that you probably will not be able to write everything you want to write within the same story without it losing a sense of coh
This is really great advice and basically what I was struggling with! I had a near comprehensive list of Marxist and liberal themes that I was trying to shoehorn into a pastiche of short stories I've written and it just... wasn't working
<3 hell yeah lol
lemme know when you find out it's been a rough year and a half 
(remind me to post a less glib/downer version of my response to this b/c it's an interesting question and topic, just in my feelings abt my lack of a consistent creative practice for the past while)
It started with a seed of an idea, in my case specifically "what if the tech billionaire ruling class actually got everything they said they wanted, including offworld fiefdoms, clinical immortality, and no restrictions on their power or inhibitions on their whims," extrapolated on what that would look like, added in the limitations of material reality (such as how ruinously costly it would be to supply an unsustainable Martian fiefdom with a poisoned a depleted Earth's resources), weighed in the class struggles of the precariat left behind on the plundered and slowly-being-killed home planet, and then I added mechas because I love mechas.