2. Read Cynthea Liu's Anatomy of a Synopsis, which not only explains the idea with simplicity and good sense, but breaks it all down for you in a handy five-paragraph template: one paragraph for the beginning of the story, three for the middle, one for the end. Only an idiot could get that wrong.
3. Remind yourself that you are not an idiot. You can do this.
4. Sit down and write the synopsis in present tense, third person, putting all the information in roughly the same order as it appears in the book. Each paragraph will take you approximately an hour. Looking over the whole thing and revising it will take you another hour. But in the end you should have a concise, fast-paced retelling of the main plot only of your novel, touching on subplots, characterization and worldbuilding only where they affect the main plot.
5. Put the whole thing in Garamond 12-point, so it fits on a single page. If it doesn't, resist the temptation to reduce the point size or the L-R margins to cram in more information. Trim the synopsis instead.
6. Celebrate! You did it!
Oh, I forgot:
7. Do not attempt to undertake this process when you are suffering from PMS. You will only end up angsting about how much you hate your book and how you suck at writing synopses, and make all your friends want to bludgeon you with a dead flounder.