Develop ideas for your book and think of what you'd like the art in it to look like. Either do the artwork yourself or hire an illustrator to do this for you. If you hire an illustrator, work to develop a good working relationship with them so that you can both get the project done with as little stress as possible.
Make sure that the story of your book is one that can give children a new way of looking at the world before. Think of historical stories or something as simple as saying goodnight to the day that has passed. Work hard to make the artwork reflect your works. Create an excellent book.
Write a query letter detailing some information about your book and your writing or publishing experience. Keep it brief and specific, but also be persuasive. Remember you are selling them a product--your book.
Work with your publisher to make the best book possible and then get it out in the marketplace. Do book signings at children's book stores or the children's departments in larger book stores.
Go to the Caldecott Medal Webpage and read through the criteria for submitting your work for consideration of the award. Prepare your submission and send it in before the deadline.
Win the award, write more children's book and hopefully do the whole thing all over again.