The difference between a pitch and the next stage of the development process—a proposal—is story. A proposal must demonstrate that your pitch has “legs” by providing a more specific outline of events, additional background on the characters, and some sample plots.
As with most every document in comics, there is no “right” way to format a proposal, but I’ve found through years of experience that this is what works best for me:
Lead with the high concept, a brief but dynamic one- or two-sentence description of the idea—
—follow that with a little more detail on the bigger picture, clarifying the who, what, where, when, how and why of it all—
—next, give brief bios of the main characters—no more than a short paragraph for each, but enough about them so that whoever’s reading your proposal can have a good handle on them by the time they get to—
—the outline, a more detailed version of “the bigger picture” that walks us gradually through the series an issue or two at a time. Provided it’s interesting to read, feel free to devote about a page to synopsizing issue one, a half-page or so to issue two, and gradually less detail for the next handful of installments. At this point, you’re still in the proposal stage, so you’re walking the fine line between showing you have confidence in your ideas and revealing that you’re so in love with them that you have every page of your opus worked out nine years in advance, which frightens every editor ever. If, as you’re writing your proposal, you’re not sure of the difference, imagine buying a car. Imagine buying it from a confident, knowledgeable salesman who demonstrates its most attractive features and then lets the merchandise do the talking. Then imagine buying it from a salesman who won’t let you leave the showroom until he explains in excruciating detail exactly how the high-performance fuel injection system interfaces with the 1.6L 4cyl VTEC-E engine and insists on giving you the 82-year history of the factory that tooled the camshaft. My point is, don’t overwhelm.
Then end your document with a fitting summary/conclusion.