For most writers, synopses are a big whingefest, basically. Woe is me! I have to write a synopsis to submit to a editor because she wants my series! Or: This horrible nasty agent requested a full and synopsis! Oh, noes! Whatever shall I do?
No sympathy from the real world, folks. Just get down and do it. I've been hashing out one for my WIP* and having loads of fun.
My method, for what it's worth? For the first draft, just write. This is the hero, this is the heroine, they meet, this is what happens, they fall in love, black moment, the end. Don't worry about how many pages it is, or whether there are subplots and secondary characters, or too many names, or whatever. Just get that structure down.**
Then, go and add more. What, are you crazy? Yes, but you still need to add more. I'm talking about emotion. The romance part. The shattereds and overjoyeds and heartbrokens. Action and reaction, emotional turning points. Make sure you're telling the story of two people falling in love. No matter what else is going on.
Then, go back in and cut it.
Cut it for excess unnecessary wordage, you don't need. Cut it by summarising subplots and eliminating repetition and stuff that's in there twice. Cut it for description and character names that don't need to be there aren't required (yeah, and using five words when two will do). Cut it for action that you don't need to explain and unnecessary plot lines that don't need to be there, because hey, this is a romance, not an action story. "They slaughter the villain and escape" is elegantly sufficient.
And then, go back to the beginning, and cut it again. And again. And again, until it meets your word limit.
I've still got 300 words to go. Snip, snip, snip...
* Before it's finished, naturally, like a good little plotter. All you smug into-the-misters can bite me, okay? Sometimes being too cowardly to pantz is totally worth it :)
** If you don't know what the structure of your book is... well, see note 1.