My advice is to just start and see where it takes you! Really, that's the way any story starts, right?
As a writer only, I just start a "vomit draft" where I get everything on paper of what I think I want the story to say, and it's truly awful. But then I start chipping away, rearranging, adding, cutting, etc. till I find my story. You have the advantage of being an illustrator so you can do the visuals exactly as you planned. So if it's more of a visual story, start with the illustrations and add words where you feel you need them.
Of course, I hope illustrators who have decided to also write will chime in as their experience might be more valuable to you.
Best of luck. It sounds interesting.