Writing a synopsis can be harder than writing the book itself. A synopsis should introduce the central characters and be a breakdown of the main plot and storyline. Sometimes it is difficult for authors to take a big step back and see the whole picture and then condense it into a synopsis.
There are many different types of a synopsis from a single sentence to an expanded version. It is beneficial to write different ones for every length possible. Sometimes it is easer to write the expanded version, and then scale back layer by layer to get to the one sentence that grasps the story. Don't forget your hook!
If you are writing to an editor, make sure to tell them every thing that happens. Yes, even your spoilers. They want to know if you have written a great story and have delivered on your hook.
Here are several great websites that will help you develop your synopsis and good luck. This was a tough one for me!