I know the feeling.
You sweated bullets and cried blood for how long getting that ms together? Your tortured brain finally performed all the necessary contortions to produce a query that is written for results. And now... you get to write a synopsis!! Hooray!!
Ok well... not so much with the hooray. I've written a few synops and I'm not going to tell you that I secretly enjoy it, or that it's not as hard as it sounds. 'Cause I don't, and 'cause it sucks.
Here's the bad news: If you take the route that BBC-of-the-past (the unagented one) took, then you'll choose to send your query only to agents that don't request a synopsis. And while that might sound attractive, it also seriously limits your playing field. (Note - BBC's agent does request a synop, good thing I got over my synopphobia).
Here's the good news: I have never heard of a writer getting signed because they wrote such a hot-damn-awesome synop that the agent snapped them up like a toad with a three legged cricket.
The purpose of a synopsis is to show the agent that you've got a plot arc, character development, and pacing all figured out - without them having to read every single word of an ms they're not sure they're sold on yet. The purpose of a synopsis is not to torture you or make you stab yourself in the eye with a carrot.
Tips on writing a synopsis? There's lots of advice out there, but just like writing a novel, you'll find that some of it will work for you, and some won't. So I'll tell you how I do it, and you can take it or leave it.
First - Make sure that your story is edited before you write a synop. And I mean edited with The Bloody Hatchet of Adverb Gore.
Second - Edit it again.
Third - Write your synop as if you were telling your story to a friend. Just write it - have a brain vomit that includes the salient points in your ms. If you hit a point where you think, "Hey, wait a sec... if I move on to the next plot-moving point, it skips like, 50 pages." Then you need to -
Fourth - Edit your ms.
Fifth - Hack away at that synop. Are you aiming for four pages, but you've got five? Should be fine - just kill some extraneous "that's" and re-check your phrasing for the cheapest word count you can get away with and still convey your idea. Are you aiming for two pages, but you've got four? Well, haul that axe back out and take a hard look at those sentences you're leaving in to illustrate how awesome you are, but don't necessarily show forward plot movement. It's called "Kill Your Synop Darlings," and it's a game with a high body count.
So now what? Ask your betas to look at it, especially those who have read the ms in question. You'd be surprised how many of them will have some insight on how to improve it. Beta readers are useful across the board - synopsis, query, novel. Use them wisely. Use them well.
And then? Put it in the envelope, attach it to the email, and send the sucker.
Then refer back to the good news.