I don't know how instructive this will be but I'll try to explain the steps as I think about a new book.
My horse books sell well so I've been encouraged to write a new one. Problem--no idea. Then I remembered, vividly, an incident from when I had my stable and was teaching quite a number of students. It wasn't just the incident, it was the situation this student was in. She may not have been particularly interesting in real life but dramatically, there is something there.
Now I have to think about whether she's the main character or is the story told through someone else's eyes. Does she have the biggest transformational arc or does someone else. What do I mean by transformational arc? Usually the main character starts at A and winds up at Z. You want to see them change and grow.
Okay say Miss Smith goes from A to Z. Good for her. What if Miss Smith makes this journey but we have another character, Miss Williams, that also goes from A to Z but for other reasons? That's another layer of complexity to the story. It also gives us an A and a B story. I like that better.
We need a C story. Do we need some comic relief? That's always good. And it should have nothing to do with the plot of the A or B story although it may intersect at times. I have a thought about this. It's a little like mud daub. When you build a wall, there are always spaces that need to be filled in. This C story can be thrown in whenever the story needs a little tweak, when it needs a respite. I'll bet the characters learn from it, too, but I don't know how right now.
No, I'm not going to start writing, I'm going to look for incidents that move the characters from A to Z.
What's Z? Plotwise, I have no idea. Thematically, I want the characters to get to a place where they have sacrificed for someone else.
Already this becomes a story that has something to say. That's not very commercial. It would be better to have no one sacrifice and someone just win the Grand Championship at the horse show. That's really easy to understand. I doubt if I could maintain my interest in the book for that long and I have to live with the thing. I'll think about dumbing it down, though.