Synopses can be tricky to write, and they cause a lot of headaches among new writers.
My main advice is to keep it short and factual. Put yourself in the position of an editor, agent, or someone employed by them to wade through piles of manuscript after manuscript on a daily basis trying to work out what the accompanying novel or sample chapters are about.
You don’t want to be irritated by clever wordplay, witty asides from the writer. You don’t even really want the synopsis to ‘sell’ the novel with lots of the kind of stuff you get in the blurb of a published book. And you definitely don’t want to be frustrated with a synopsis that ends with How will our hero get out of THAT?!
What you want is facts: what is the novel about, when and where is it set, who are the main characters, and what are the main plot points? You want to know that ex-SAS man Dan Granite kills former Russian KGB agent Vladimir Duemova in Chapter 12 – but you don’t need to know he does it with a Heckler and Kock MP-5A3 set to fire three-round bursts. You want to know that Roger finally kisses Genevieve in Chapter 6, but not that he did so after visiting his aunt Matilda’s house to feed her cat because she’s away on holiday in the Algarve. Stick to the stuff that moves the plot forward in a significant way.
For most novels it should be possible to get the essentials down on a single page – and increasingly, publishers and agents are demanding this anyway. (But note that synopses, unlike the main manuscript, are single-spaced.)
For you non-fiction writers out there, it’s important to understand that the way the word ‘synopsis’ is sometimes used in this field can be misleading to someone coming from a fiction background. What is usually meant here is a chapter outline: a chapter-by-chapter breakdown of the whole book, whether or not you have actually written it yet (non-fiction writers can and do approach publishers at an earlier stage than novelists). In my experience, although fiction writers sometimes provide chapter outlines they are rarely required. The same goes for contents pages, which, when you think about it, are virtually pointless at the submission stage of things.