First off, the format of a synopsis should be the same as for any manuscript. Keep it consistent and easy on the eyes.
An editor needs to know that the author knows the novel has a beginning, a middle, and an end. The synopsis must show this. Some editors require a synopsis be submitted along with a complete manuscript to verify that the author has control of the story. The two documents need to match. Once you have sold one or more books with a complete manuscript, you will sell on partial manuscripts consisting of a synopsis and the first three chapters.
Again, your synopsis needs to demonstrate a beginning a middle and an end as well as a flow of action. Never keep secrets from an editor. Reveal the ending as well as the solution to any mystery in your plot.
Writing a synopsis for yourself, sometimes several different ones through the course of completing a book, helps keep you focused on the action/reaction and consequences. If your characters wander off in unexpected directions, this will change the ending of the book. A new synopsis helps you figure out how to make it stronger than your first stab at it. Or if you are tied to the first dynamic ending you envisioned, a new synopsis can guide the action toward it with new twists in the middle.
In a synopsis you may tell rather than show, and use present tense rather than past. A few snappy bits of dialogue can make a point in the synopsis. However, it should only make a point and not demonstrate your ability to write snappy dialogue.
Synopses need not be boring. Keeping it interesting and concise is an art form that requires practice. I have been known to take as long on the synopsis as I do to write the first three chapters or thirty pages of the manuscript. Sometimes longer.
The length of a synopsis varies with its purpose. The one that goes with a query letter to interest an editor or agent should be no more than two pages. Maximum. Cutting a long synopsis down to two pages is HARD. A more efficient method for me is to start with a back cover blurb, 100-150 words that names your protagonist, his/her life goal, the conflict to that goal (antagonist), and the emotional growth the protag must go through to overcome the conflict and achieve the goal. Expanding from that can make a concise two pager that works.
The longer synopsis that goes with the chapters or full manuscript depends upon the editor. Some editors want roughly one page of synopsis for every twenty-five pages of book. Other editors prefer to keep it under ten pages. Ten pages is a nice average. The synopsis you write for yourself can be whatever length you need it to be. I have known some writers to produce a one hundred page synopsis. To me that is more like a rough draft. I prefer mine to be between ten and twenty pages. The partial I just submitted to sell a new series was nineteen pages plus thirty pages of chapters. The ones I create to sell additional volumes within a series vary from two paragraphs to twelve pages, depending upon how much world building I need to include. The following format works for me. Feel free to adapt it to your own style. Every writer has their own preferences. The purpose is the same.
Paragraph #1 (optional): a hook.
Real men donít watch birds for a living; real women donít ride Harley Hawgs wearing full leathers.
You can also put in a few words about the setting: alternate now, historical Russia, stock fantasy medieval, or whatever.
P#2: Protagonist. Show the characterís goal, conflict to that goal, and emotional growth necessary to overcome the conflict and achieve the goal (sound familiar?). Physical descriptions are not necessary, unless they are part of the conflict, i.e. a handicap or unacceptable cultural bias against certain features or coloring. Include professions, age, family situation, only if they help define the character.
P#3: Secondary Protag. Same as above plus their relationship to the primary protag.
P#4: Antagonist. Same as above.
(Use as many character sketches as you must. Try to limit to primaries or Point Of View characters.)
P#5: Back Story. This is a short recap of what brings us to the action that begins on Page 1 of the book.
P#6-P#20 (or however many paragraphs you need to tell the basic plot): Plot.
Recount major plot points and transitions. Remember to show characters reacting to action and growing. Do not forget that the book has a middle as well as a beginning. I often go to my plotting tools and pick out the scenes that demonstrate the stages of the heroís journey as identified by Joseph Campbell.
P#21: Crisis. This is when the world comes crashing down around your protagís ears.
P#22: Dark Moment. The characters have to hit bottom in order to compromise or consider doing the unthinkable to defeat the villain. Out of the frying pan into the fire.
P#23: Climax. This is the final twenty minutes of murder and mayhem at the end of the movie.
P#24: Resolution. Tie up loose ends, or as many as you can and still leave an opening for the sequel. Keep this as short as possible. This is the wedding at the end of the romance, not the rest of their lives together.
With a little practice, writing a synopsis becomes a familiar and comfortable part of the process of creating a novel.