So that's the first thing. Although you're not going to illustrate it yourself, you have to be able to visualise how the finished book might look. When the book is open, you'll be looking at two pages at a time, the left and the right page. That's called a spread. Books are made by printing on enormous sheets of paper which are then cut and folded into 32 pages. (Longer books are made by glueing those 32 pages together - if you check the spine you can usually see the folds.)
With a picture book the first couple of pages will be taken up by a title page, and the publishing information page, the end pages will be perhaps more information on other titles. You have between 24 and 28 pages to play with, which translates to 12, 13 or 14 spreads. You have to work out how your story fits into those spreads - you might find folding up some sheets of paper to make a dummy book is useful.
Then you present the text in spreads....
Fergus lived with his mother in a cave on the edge of the deep dark forest.
One summer evening, Fergus was busy chasing butterflies.
'Time for bed,' said his mother.
'No, it isn't,' Fergus said, jumping higher. The day was still bright outside the cave.
'Time for bed,' his mother said again, scooping him up.
But Fergus wriggled out of her arms.
'I'm not going to bed,' Fergus said. 'I'm going to find an adventure.'
And he ran from the cave out into the forest.
The trees were tall and made long shadows across the path, but the leaves danced and rustled in the breeze.
'I don't need a mummy,' laughed Fergus as he skipped along. 'I'm going to have an adventure on my very own.'
(Illus: Mother Bear is in the background, hiding behind a tree)
(Illus: Dusk is falling)
And so on, keeping each spread together ie don't split them between pages. Add a word count and your contact details at the end, and you're done.