We're going to assume that you don't have a contract that requires you to submit your synopsis before you write the book. (Don't scoff: plenty of authors are required to do that. It all depends on the genre, contract, publishing house, etc..)
So where do we begin? Well, you've written a book. You know what it's about, right? Now here's the most important question: What's the character arc? That's right, you need to put in character arc as well as story arc. Remember, the protagonist may not be the character who arcs, but if you're writing commercial or pulp (pop) fiction, chances are the protagonist IS the arcing character. Now if you wrote an outline before you started the book, good. That'll give you a great place to start with your synopsis! If not, don't worry. Most of the steps are the same. Now to begin!*
Step 1: Make sure you have your favorite drink available. I stick with coffee because of its stimulating effect. Compulsively drink said liquid until it's gone. Stare at your computer screen, looking back and forth between the blank document that will be your synopsis and the novel you just wrote. When you realize your cup is empty, refill it. If necessary, run to the store to replenish your supply. Stare at your computer some more, and the write the first paragraph that describes your story. Make sure it's in present tense, third-person POV! That's the ONLY consistent rule for writing a synopsis.
Step 2: Hurray! After drinking so much liquid that you've had to go to the toilet a dozen times in three hours, you finally got the first paragraph written! Does it accurately convey the action of the first opening scenes? No? Don't delete it! Keep it on the page, write a new paragraph, using some of the ideas you write above, and when you're satisfied, THEN you can delete that first paragraph. Now I realize this has been a difficult day, so turn off your computer. It's time for bed. Pick it up again tomorrow.
Step 3: Sleep in. You got a lot done yesterday! When you're up, repeat step 1. Now it's time to finish that synopsis! This is the easy part. You're just going to describe the MOST important scenes of the plot and how the arcing character feels about them. Make sure the character's change is clear. Pepper in a few colorful phrases to make sure the synopsis isn't boring, but try to keep word-count to a minimum. (Note: Industry standards vary. Some agents and editors want one page, single-spaced. Some want one to two pages double-spaced, and some want anywhere from three pages, single-spaced to eight pages, double-spaced. There's no standard, and only five or six agents I've run across in the last four years even mention what they want in a synopsis. Don't stress about it, just do the best you can).
Step 4: YAY! You got that ENTIRE synopsis done. It only took you two days, too. Now it's time to send it to your writing partner/critique group/beta readers. Enjoy this, because this will be the best feeling EVER. Until you get your feedback.
Step 5: Bang your head against a wall, door, table, counter-top, or any other handy surface. Try not to cry TOO much because swollen eyes are difficult to see through. You need to see because it's time to throw out all of your hard work and start over.
Step 6: Repeat 1-4 until your feedback is EXTREMELY positive. Avoid step 5 as much as possible.
I hope that helps.
*Disclaimer: I am not an expert at writing synopses. Do not – I repeat – DO NOT use this advice unless it ACTUALLY helps you. I assume ZERO responsibility for any consequences that result from using this guide. On the other hand, if this is a successful method, I expect to see my name in your acknowledgements section. Thank you, and have a great writing day!