Step One: Write one sentence describing each of the following: hero, heroine, beginning hook, major turning points and resolution. You should have (depending on length) 7 sentences, give or take. I write shorter so I usually have 3 turning points; obviously longer books will have more.
Step Two: Expand both the Hero and Heroine's sentence to become a GMC statement; 3-4 sentences (max) is good.
Step Two: Expand the beginning, turning points and resolution sentences to be NO MORE THAN a 3-4 sentence paragraph. No cheating: don't use And But Or to conjugate two sentences. This needs to be succinct.
Step Three: Add in the fun - what leads the characters from Beginning to Turning Point 1 to Turning Point 2 to the Black Moment to the Resolution? Again - be succinct. Some of these will be one sentence, some will be 2. Very few should be more than 3 sentences long.
Step Four: Don't forget the emotion. You've done such a good job of getting the Who What and When and possibly How down on the page. This is where you add in the Why - why does the Heroine act like ___, why does the Hero respond like ___; what emotional entanglement leads them to ___. Once again, be succinct. You're dealing with emotions at this point, but remember it's a synopsis not the actual book.
Step Five: Read it. At least twice, looking for moments or reactions or happenings that you forgot to include. If something big - not just the heroine getting a manicure, I mean a moment that changes things for the hero or heroine - happens and you've not mentioned, this is the time to add it in.
You're through. That's my simple synopsis technique; usually I can keep this to 2 pages. When I remember it's a synopsis and not the actual book. :)
Do you have a secret formula?