It
is frustrating when they ask for one but they don't give you specifics. In that case, you can't really go wrong with a basic one as Jane Friedman outlines in that blog article.
Evernight asks for a one-page synopsis, so I have that down to a science now.

In fact, I usually write it once I've got the first 10K or so done on the story. It helps me stay on track as I write the rest of the book, and then I can simply revise the synopsis when the story takes a different turn. Translation: when my characters have their own ideas about where the book is going.

But this way, it's done when I'm done with the book and ready to submit it.
Siren-BookStrand wanted no more than 300 words, which was way more challenging. It had to be very bare bones. "They met, they kissed a few times, she hated him, he pushed to keep seeing her, they finally had sex, it was kinky, they fell in love, the end." LOL!!
