The best picture books ware simple, yet somehow profound with the stories they tell. Think of Dr. Seuss' books; they were always very simple narratives, but the ideas  used carried a lot of weight. Think of a high concept that involves something that would appeal to a wide variety of ages.  Even if you feel tempted, try not to turn the story into a morality tale. Very few readers are going to be genuinely interested in reading a veiled lesson in etiquette or behaviour. If you're more of an illustrator than a storyteller, you can always  illustrate an existing story. There are countless picture books on the market that are based on classic fairy tales. Inspiration for stories can be found by immersing yourself in media. Films, music and books are all willing templates for your own stories. Most stories need interesting characters to populate the action with. Having spent some time mulling up a plot, the characters should come relatively naturally. While you should have a basic idea of the role each character will inhabit in the story, it's important to give them each a personal flavour. The best characters are easy to imagine having a life of their own beyond the confines of the narrative.  While you're concocting characters, you should also be thinking of the way they'll look in your illustrations. Visually drab characters with a rich psychological profile probably aren't suited for a picture book. Animals are very popular to use in children's picture books. Animals have a universal appeal, and anthropomorphizing them to fill human role makes them less offensive to some readers. Speaking generally, animals are also more interesting to draw. Using a word processor, write up the story as you see fit, dividing it into a clear beginning, middle and end. At this stage of writing, you needn't worry about the proper choice of words. You're just trying to get the ideas into a basic framework. From there, you'll be able to add a writer's tone and boost your wordplay. Keep your word count around the 500 word mark. Anything more will be difficult to fit into the book and will distract from the illustrations. It's best to be tactical and efficient with your word choice. With your narrative all written out, you need to divide it into as many pages as you have for the story in your book. Include at least one action per panel; anywhere from one to four sentences per page should be great. It will be much easier to edit out your work now that it is separated into smaller sections. Focus on one section at a time, and transform the template you have into writing with style and form. While the specifics will really vary depending on your tone as a writer and the subject matter, keeping it concise and poetic is usually essential for picture books.  Use simple, effective language that benefits the illustrations you'll be including. Simple rhymes are helpful, but don't build your writing around them specifically. A mediocre rhyme is worse than no rhyme at all. Alliteration is an easy trick, and makes the writing sound more melodious.

Summary:
Outline your story. Create some characters. Write a rough draft of your story. Divide your draft into pages. Edit and finalize your draft.