"This is the charming story of a box turtle whose life begins in a New England forest in 1890, and who lives over 100 years and witnesses changes to the landscape that threaten her habitat until the spirit of conservation transforms her remaining woodlot. A story that teaches, conservation, wildlife biology, and history."
So here's a bit of the process. I start with thumbnail sketches. Each rectangle represents two facing pages. There are 32 pages in your average picturebook. 

I'm playing with how to best illustrate the words. And also working out composition, where action/consequences fall, direction of movement, and how the facing pages will complement one another. No one sees the thumbnails but me.
On the left is a closeup of one of the spreads.

 Below, is from the next step - book dummy. Dummies are still sketches, but more detailed. It's what I send to the editor and art director. You can see there's a bit of a change on the left hand page.
And here's the final illustration. The words are put in later by the designer. I work in Adobe Photoshop on a Wacom tablet. This spread took me about a week to finish.
My cat, Chloe, makes a cameo! Always fun to do this kind of thing.