I’m going to use the fic I’m planning to write during NaNo this year as an example here, but my process is pretty much the same for any story I write. This particular story was sparked by a prompt from a Twitter account I follow, au_idea_bot, the prompt in question being incredibly long cross-country trip au. I really didn’t want the prompt to give me any ideas, because I have way too many WIPs as it is and my track record for actually finishing stories I start during NaNo is absolutely woeful.
My brain latched onto it anyway. Not only did it latch on, it dug right into my memories of a camping/road trip my family went on around my home state in January 1992 to visit my great-grandmother, right before I started second grade. Which meant one thing in particular - this story was going to be semi-autobiographical.
Next, I had to decide what the prompt’s definition of long was - time or distance? I live in Australia, so for me (and probably many other Aussies) going on long road trips is pretty normal - I think nothing of driving a few hundred kilometres to go Christmas shopping in the nearest city, for example. The trip my family went on when I was a kid, according to Google Maps, is nearly 2,000 kilometres from start to finish, and took us two or three weeks (I went back to school on time that year, so definitely no longer than a few weeks). I ended up deciding that the midpoint of the road trip in the story was going to be clear across the other side of the country to me, on the west coast in Perth, and that the trip was going to take my characters all around the Australian mainland. So the road trip in the story was going to be long in both time and distance.
Now that my idea was nailed down, I had to decide on my characters. I generally write RPF for the band Hanson (usually with the lead singer, Taylor, as my focus), so that was one character sorted already. Next I had to decide if they were married (they were), did they have any kids (two kids - a girl and a boy), where they lived (a few hundred kilometres up the coast from me), and why they were going on this road trip (visiting family in Perth). I named my original characters (Maddie, Alinta, Luca, Melinda and Tony), figured out why Taylor lived so far from the rest of his family (he grew up in Perth but went to university in Sydney, where he met Maddie), and his job (music teacher at a local high school, and runs a surfing school on the weekends and in school holidays).
That’s pretty much what I have so far. Right now I’m working out the route of the road trip in Google Maps, which apart from working out a summary is really the last major thing I need to do. I still have plenty of time left, so I don’t think I’ll have much trouble getting things done. If it turns out I need to do more after I start writing (which is admittedly par for the course with me, as I’m very much a plantser), then I’ll figure things out as I go.