Here's the thing, after a great deal of research, it seems that a lot of agents still ask for a synopsis and not just a query letter (or actual pages). So, as well as working on editing and editing and re-editing the query letter (and still editing the novel), I'm also writing a synopsis.
Google was particularly unhelpful when I asked it how to do this. It was super helpful with the query letter, so I was rather disappointed by this. Most of the links that came up said things like 'there's not one way to write a synopsis' or 'everyone has a different way to do this' or 'it's hard to give advise on the dreaded synopsis'.
None of those filled me with much confidence. But after a bunch of sites, it became apparent that even if there is not 'one way', there is a bunch of things NOT to do, and there's a few things that YOU MUST do. Unsurprisingly, neither of these lists were as clear-cut as the query letter ones. Are you getting a theme here?
One of my favourite things on the NOT to do list was: don't make it sound like a story. And for a while that confused me a lot, because it's the synopsis of a story, how can it not sound like a story? But after reading a bunch of sample synopses online, I realized what they were getting at. It's supposed to be boring. It's supposed to be 'this is what happens' and then 'this is what happened afterwards'.
That doesn't mean it has to be dry as dust or just a statement of events. One of the best pieces of advice Google gave me was to remember to put emotion into the synopsis. The story is all about how the characters feel and react to the events that happen, and the synopsis has to convey that. So it can't just be all 'Character X goes to a party and meets character Y.' 'Character Y is a bad boy and isn't good for her'. 'Character X falls in love with character Y.' OMG I'm bored already. There's no emotion in that. There's no story in that. It shouldn't sound like a story, but it should clearly show that you've written a story. A story is not just a bunch of things happening. It has to have emotion.
Now how to show that in the synopsis. Well, I'll get back to you, shall I, because I'm still writing it. Like the query letter, it's a work in progress. It takes time. It takes a lot of rewriting and reworking. It takes a lot of editing. And a lot of having other people read it and tell me how wrong it is. I'm not sure I'll ever be 'happy' with it, but I will reach the stage that I have to let it go. And hopefully, at the end of the day, the query will make the first impression...and the synopsis won't blow it too badly.