The synopsis may be the most challenging thing you will write as a writer. The synopsis is a one-to-five page summary of your whole book. Different agents will request different page lengths, but a one-page synopsis is the most commonly requested. It is worth having three different versions (one-page, two-page, five-page) on the occasion you might need to send a longer one to a different agent.
RULES FOR A SYNOPSIS
A synopsis should provide the entire plot. You do not need to go into elaborate detail for your synopsis; it should be a broad overview that outlines the problem, inciting incident, climax, and resolution. You should introduce only the main characters who are relevant to the plot and their goals and setbacks. It is standard to capitalize the names of the characters and put their age in parentheses next to their name. The synopsis should match the tone and voice of the writing of your novel, too.
Additional information about writing a synopsis can be found at the links below.
Traditional Publishing Blogs
PART 3: The Synopsis