How to write a synopsis

Because the synopsis has to cover a lot of ground in a very short number of pages, it must be concise and free of unimportant details. For this reason you want to avoid long paragraphs of description. No one is interested in what your characters look like at this point unless the character has a certain physical feature or flaw that's integral to the story. Avoid inserting dialogue or passages from the manuscript to elaborate on the synopsis elements.
The synopsis is meant to convey a finite set of elements -- the ones you've already drafted in your previous assignments. It should state where and when your story takes place, but not the genre and word count. That information is for your cover letter and on the title page of your manuscript.
A synopsis is always written in present tense, usually in third person. However, if you've written a first person book, you may choose to write your synopsis in first person. That's perfectly acceptable.

A synopsis should not be a dry read. The best synopsis is one in which the author has been able to instill the voice used in her manuscript into the synopsis.

So now you need to take all the information you've created about your characters and plot and put them together into what will become your synopsis. This will involve a little shuffling of the various elements you've created and also include some of the information from your query letter.

1. Start with the High Concept or Log Line you developed for your book and the central question of your story.

2. Introduce your major characters, giving their goals, motivations, and conflicts. Tell where and when your story takes place. Avoid detailed descriptions.

3. Add your plot points from what you created in your 3-act structure.

4. Go back and develop each element into tight, compelling sentences and paragraphs that capture the tone and voice of your manuscript and flow from one element to the next in a logical order. This may mean shuffling the elements around, as well as combining some within the same sentence or paragraph.

5. Rewrite each sentence and keep rewriting until your synopsis is as tight and polished as possible. Make every word choice and every sentence count. Use specific adjectives and verbs rather than general ones. Avoid unnecessary descriptions, minor details, and rambling sentence structure. In other words, write your synopsis in a style and voice that compliments the writing in your manuscript.
Once you've written your synopsis, it's time to format it properly. The synopsis is formatted exactly like your manuscript pages with the following exceptions:

-- There is no need to start 1/3 - 1/2 down the page.
-- Include the title of the book, "synopsis", and the author's name centered at the beginning of the synopsis.
-- Include "synopsis" in the header with the author's name and title of manuscript.
-- The first time a character is named, type the name all in caps.

Length of the synopsis varies. Some editors and agents prefer 3-5 pages. Some prefer more detailed synopses of around 10 pages. Some like a page for approximately every 10,000 words of your ms. If the editor or agent doesn't specify a length, you're usually safe with something around 3-10 pages. Don't pad the synopsis. Include everything that's important but leave out everything that incidental. If your ms. has a strong secondary plot, include it. If the subplots are minor, don't include them. Also, to avoid confusion, if you find it necessary to mention secondary characters, refer to them by their relationships to the protagonist (sister, cousin, teacher, etc.) and not by their proper names.

-Lois Winston