These are some of the first images I did for Book 1. This spread as a whole didn't make it into the final book, but it got me going in the right direction.
Of course, I am also writing a full-fledged novel here. So while I'm thinking about the kinds of images I might want, the primary thing I am doing is making an outline of the key beats of the STORY. (SEE my page on "Story Tips" for a full explanation of all that). What I'll add here, because it is SUPER SUPER SUPER important.....is that I start off very SIMPLE. Meaning with one sentence, with what the story is basically about.
For example, the first book of Third Grade Mermaid is about a young mermaid who dreams of being one of the beautiful "Singing Sirens" on her school's swim team, but first she has to pass something not quite as glamorous: her Spelling Test.
From there I just start beating it out, answering the key questions: What does she want? Why can't she get it? What does she do to succeed? How does this fail...multiple times? What kind of mermaid-like issues will she run into? And of course, how does it end?
The tricky part is taking the time-tested formulas of story, and creating something entirely yours. For that I don't really have advice outside of being yourself. Don't copy someone else's style. Don't try to "sound" like anyone else. And don't be afraid of doing it "wrong." As far as I can tell, Editors and Agents are not looking for another John Grisham or Stephen King....because they already have the real thing! So again, maybe build your story with the kinds of blueprints that have proven to stand, but paint the walls a color no one has ever seen.