You will first want to set up your work space with a nice large sheet of paper, a pencil and a good eraser. Then draw out all the pages on your paper like the storyboard layout shown here for a 32 page book. Most children's books are printed and cut into 32 pages. Grab a picture book and take a look at the first 1-5 pages, they are flexible in terms of how you want to handle them. See the storyboard by enlarging it here, and you can determine how you want those pages to layout. For example some storybooks will have a half title and a full title, where some books will only have a full title page.
Then take your manuscript and look at it in terms of page breaks. A page break is where you determine where you break your story copy up according to pages. I print out my manuscript very small and start cutting it into those page breaks. If your text is very minimal you can just pencil it in. Now my story is guiding me through the pages. This has sometimes helped me to clarify my story. I have done even more editing from this process.
The next step is to "scribble sketch" drawings (your picture ideas) on each storyboard page. I let my words guide my art. I loosely draw out my ideas. I like to scribble in circle and square shapes. Please enlarge my illustration to see. You will see your book start to take form.
I create two separate storyboards. The first one for only my text and the second one with just the rough pencil drawings. My next step is to draw all my pictures tighter and scan them into the computer. Working with a publishing program on the computer I marry the text and the art following from my storyboards. At this point I design how my text will look on each page. I like to work with a standard 11" x 17" paper. I print all my pages out in spreads to fit this size. Spreads are two pages facing one another, pages two and three, and so on. The left is always even numbers and the right is always odd numbers.
My last step after the spreads are printed, I combine all into a book dummy. I fold the 11 x17 in half. Note: You do not have to print 11x17 you can print any size you want but remember you are printing in spreads, folded into a proportionate size of your book. Using double sided tape or glue stick I take my cover, folded in half and tape it to the inside front cover (back to back) and page one (it is the opposite page from the inside front cover) is taped to page two (back to back) and page three is taped to page four (back to back) so on. Folding and taping all pages until it becomes a book dummy. Also note: I do not bother showing the first 5 pages in my book dummy. If I follow my storyboard I know I will have a 32 page book. This is then ready to send with the completed manuscript to a publisher or publishers of my choosing.