The title to the featured book says it all. "Writing a synopsis is tougher than writing the entire novel." I've just finished mine for my historical A Man for Annalee. Normally I wouldn't write it until I'd finished writing the book, but I needed to include a synopsis along with my first chapter entry to two different writing contests.
Formatting a synopsis is similar to formatting a novel. One inch margins all the way around the document. Book title on left side of header and page number on the right. Use Times Roman or Courier New fonts, size 12. Unlike a manuscript, though, a synopsis begins at the top of the page and is single spaced.
A synopsis takes a 95,000 word manuscript and shrinks it into two pages. Some editors will accept a five-page synopsis, but are especially thrilled if you can do it in two. They're extremely busy and stressed; their time is premium. So, the shorter you can make it, the better your chances of getting on their good side.
Now, when you're as wordy as I, shrinking a novel into two pages is like stepping out of a shower and, without drying off, squeezing a size-20 body into a size-12 girdle.
A synopsis demands that you tell everything that happens within the book. There is little room to showcase your style, so chose your words wisely, for you are literally saying, "This happened and then this, then this, followed by that and that, and then this." Arrrghh!!
The first time you mention a character, that character's name must be written in capital letters. From that introduction on, you may type it normally. Show what emotions drive the main characters. Show how the main characters change and grow. Tell the ending! Don't make the mistake of thinking you'll entice an editor to read your book by leaving your ending a mystery. You won't. What you will do is antagonzie the editor and probably ruin your chances to have your complete manuscript read. Include the genre and word count.
Read and reread it. Share it with your writers/critique group. Give them the chance to tear it apart. That's exactly what my writers' group did last night. I have to admit that it wasn't a pretty scene, but ultimately they helped make my synopsis stronger. I rewrote it three more times today before submitting it, along with my first chapter. I must admit to a strong sense of relief that it's written--it is a necessary evil to the writing process. Any potential agent will require one. All editors insist on reading your synopsis; make it shine.