Outline your story plot. What are the key elements of the story? Where does it begin and where will it end? What turning points are important enough to define the end result of the story? You can't pitch a story proposal to any publisher without knowing the story you plan to tell.
Write a synopsis. A synopsis differs from an outline. While the outline defines specific plot points of your story, the synopsis gives an overview of what your story is about. The synopsis should be two or three paragraphs in length and tell the publisher about your story.
Write a query letter. Address the letter to a specific editor. If you can't find the appropriate editor in the submission guidelines for a publisher, address the query letter to "Acquisitions Department." Start the letter by telling the editor who you are. Give a brief overview of any writing credits you have then let the editor know you have a story plot you'd like them to consider. Let them know the name of the story or novel and give them an approximate length in word count.
Drop down to the next paragraph in your letter and write the short synopsis of the story. Don't try to keep the ending from them as a surprise. Tell them everything. Save the surprises for the readers.
End your query letter and plot proposal by thanking the editor for his or her time and letting them know you look forward to hearing from them. Don't go on about how awesome your idea is. It's your idea and you probably wouldn't be presenting it if you didn't think it was awesome. Let the proposition speak for itself.