First consider this: As much as you'd like to write the next Velveteen Rabbit or perhaps tell the true life story of Ella Fitzgerald, remember that a longer text means an older audience. Who do you want to be reading your book? A toddler?A kindergartner? Or a kid who's going into the third grade?
There are picture books for the older audience, but from my bookseller experience, most kids who are 7 and up are reading chapter books and novels––the next Junie B. Jones or for the more advanced, delving into the next Harry Potter.
Shoppers are generally looking to buy picture books for the younger group. Most three year olds cannot sit through a picture book that consists of 3,000 plus words! So try to keep your story under 1,000 words and edit, edit, edit!
Now you have the perfect text and it's just right for your target audience. Now what? Well, you're an illustrator, right? So I'm sure you want to get illustrating! But wait just a minute. Are you sure you know HOW to illustrate a picture book? Do you know how many pages to make it? Do you know where to place the text? You're not thinking of illustrating the whole book in full color before you submit, are you?
First things first... research!
Go to your local library. Look at all the books. What catches your eye? What doesn't?
Go to the bookstore. What's on display? What isn't?
Go to a local story time. See what books are read. What age kids are listening? Are they paying attention? What books do they respond best to?
Once you've discovered some of your favorites, take them home, look at them... study them! Who published what? How many pages do the books have? How many words?
Sketches
Work in a way that's best for you.
DO make sure, however, that your sketches clearly convey the characters, action, and setting. They can't be so rough that the editor and art director don't know what's going on.
It's best to make sure that the action (characters running, cars zooming and so on) goes from left to right, encouraging the reader to turn the page. This is not a rule but it's good to do so whenever possible.