I've published 7 novels now, and 2 of those were first submitted as a synopsis (Blindsided and My Little Brony). For my ninja sequel, my editor asked me to write synopses of ALL the ninja series so she had a reference to keep in mind as she worked through edits. Just this last week I wrote another one for a potential writing gig I want. Whether the synopsis is for a sequel or a work-for-hire or just reference, they are an important thing to know how to do. They can be the difference between selling your whole series or just one book. They can get you a job as a packaged book writer.
So, how do you write these synopses things? Well, first you need to know what a synopsis must and must not do. Then the rest is up to interpretation.
1. Your synopsis needs to demonstrate that you know how to plot a novel.
This is really the major thing an agent or editor is looking for when they request a synopsis. If you don't have a strong enough understanding of plotting—it will show in your synopsis. You will choose the wrong things to focus on or the plot points will be clearly weak.
If you feel like you can't write a synopsis, my number one tip is to learn more about plotting. Read Save The Cat. Check out Dan Wells's 7-point Plot Structure. Brush up on the 3 Act Play set up. Dissect your own plot and the plots of other novels/movies/television. A synopsis will reveal all the holes in your story's plot, so use that as a tool to gauge your novel's strengths and weaknesses.
2. Your synopsis must not be boring.
This is more difficult than the average person realizes. You must reduce your novel to the bare bones, and then make those bones sound super interesting. How do you pull that off?
Focus on the stakes, emotions, and complications for your main character that push them through the plot.
Don't strip your synopsis of voice.
Use active verbs to push action forward.
3. Your synopsis must be focused.
Your novel will likely have way more characters and subplots and settings than you can put in the synopsis. This is okay. It is not a novel, it's a focused look at your plot structure. If you don't need to mention a character's name, don't. If you can skip a description, do. Look at your novel as a whole and pick out the most essential pieces. These will likely be your main character and their motivations, the plot twists and turns, and your villain and their motivations.
4. Your synopsis cannot be too long.
While this is a longer version of a query in a way, your synopsis should not be 12 pages long. At least not one that you expect anyone to read with interest. around 1000-1500 words is about as much as you can get away with before it just starts to feel like too much.
And that's the basics! I know this doesn't look like a lot, but these are the most important things when it comes to a synopsis. They certainly don't have to be perfect—they are mostly requested when an agent or editor wants to see if the book progresses like a well-plotted novel before taking a closer look. If you are struggling, please feel free to ask me more specific questions in comments.