Do your homework and gather a good selection of successful picture books to find out what makes them work. Your job is to read each story and analyze each page. Spot the techniques the author uses to encourage a child to read on. How is the plot entertaining? Read the story out loud and discover if it flows well or if it is awkward to read. Look at the language. Are the words simple or difficult? Is there repetition (of theme, word/phrase or sound)? If so, is the writer using it to good effect?
Make up some stories in your head and tell them to your own children or borrow some from friends, if necessary. Discover which plots work best and what characters the children respond to. Change your story in response to the children's questions and reactions, and fine-tune it. Try to include some humor or at least some funny moments.
Choose one of the most successful stories and begin to write it. Remember to include plenty of drama and a conflict which is resolved at the end. Both the subject matter and characters should appeal to children. Animals -- just think of "The Very Hungry Caterpillar" -- and other young children who have adventures are good choices. A lot of description will be unnecessary because this is the job of the pictures. For example, writing about a boy with a red hat is redundant if the picture will show that. Keep your sentences short and make sure the words flow and the story reads well aloud. Tape your story and see where you could improve the rhythm. Sometimes judicious repetition will work well.
Write each page and be aware of the pictures even through you will probably not be drawing them yourself. If you think it would be useful, include notes for the illustrator on some of your ideas for the pictures.
Make a dummy of your picture book. Work to a 28-page format for the text and illustrations.