Check with the publishing company to whom you will be sending your manuscript, to find out what they require in a synopsis, as there are some variations.
Write your synopsis in narrative form and in the present tense.
Being clear and concise, introduce the main characters (protagonists) and the conflicts they will be engaged in, whether those conflicts are emotional or physical. You need to make the editor feel as if he knows the characters and can relate to them. Brendan Nolan writes that you should "weave the characters together throughout the narrative."
Make sure your synopsis incorporates the major points of the plot and the key scenes that make up the "meat" of your article or book. Include the chief conflicts and how they are resolved. How you describe the ending of your article or book in your synopsis can make the difference in your odds of getting published.
Avoid writing a synopsis that goes chapter by chapter. Brendan Nolan warns that such a synopsis will "read as a lifeless recital of the book's content and could result in a tired publisher turning away to read a more engaging synopsis from another writer."
Still, make sure your synopsis covers the entire magazine article or book to its conclusion. Write your synopsis in such a manner that an editor or agent will want to read it to the end and will want to see your finished manuscript.
When writing your synopsis, keep in mind that editors and their agents receive dozens of manuscripts daily, and that your synopsis needs to catch their eye and be engaging, well-written and complete.