The way I plan illustrations is that I first write the children's story. I then estimate how much text fits into a page / opening. Then I read the story and mark in it what kind of a picture is needed to clarify the story. And then I list the pictures in a notebook, and write sketch / draw / paint (each with a box to tick next to them). Three to-dos per picture.
After that I look at my calendar and try to figure out how much time I have to use for the painting process.
If I have a deadline, I divide the number of days I have with the number of pictures I need to paint. Easy to see what needs to be done and when. Obviously I leave a week or two free before the actual deadline because life happens and I can't use all the days I planned for painting.
Also if I manage to finish a picture before its individual deadline date has come, I enjoy the free time.
(Below is the first pic I drew to Mr. Mummific's book "How I became a Mummy". He complains here about inviting skeletons for dinner - messy business because they are constantly hungry because all the food falls right through them to the floor. The family dogs are quite happy to see them, though, and eat so much their wrappings burst.)