One of the most daunting things a writer must do is summarize their book. A synopsis can run anywhere from one to 10 pages, although the single-page synopsis is really more of a contest creature than an agent or editor’s tool.
It’s important enough to the writer, though, to work on the synopsis throughout your projects. Once a month is a good idea, or weekly if you’re a blazing-fast novelist. You will find the synopsis changes. You might even see some relationships and connections by writing your snyopsis you did not spot during the writing of scenes and chapters.
Advice on synopsis is ample on the Web. Absolute Write, a Web site for writers, includes several articles on how to format, what to be sure to include, and what not to do: leave questions unanswered. To get one writer’s get of rules, look at an article called Mastering the Dreaded Synopsis.
A synopsis can grow from a book blurb. You can be writing the text that will go into the inside left flap of your jacket cover throughout your project, too.
I also recommend the book The Sell-Your-Novel Toolkit. It includes a great section of nine different synopses, with a breakdown of each.