If you'd like a primer on how to write a synopsis, see my posts here and here. And if you want your synopsis critiqued on this website, fill out the form here, or email your 1-2 page synopsis to me at firstname.lastname@example.org, and I'll post one critique per week (NOTE: I'll email my critique to the author as soon as I'm done, so the author won't have to wait to see his/her synopsis on the site). Thanks for participating!
My usual one-line tip for writing a synopsis has evolved from this Lewis Carroll quote from ALICE IN WONDERLAND: "Begin at the beginning ... and go on till you come to the end: then stop." This is fine advice for manuscripts that progress chronologically, using a traditional format. But what if you've written a book that jumps back and forth in time, or is unclear about the timeline, or has two or more viewpoint characters on different timelines? What about if your novel uses storytelling tools other than straight prose? How do you account for these unusual novel structures when writing your synopsis?
The point of a synopsis is to tell the reader what happens in your story. That means, even if your manuscript is not structured in a traditional, chronological manner, you still have to account for what happens in the main plot during the beginning, middle, and end of the story. In other words, don't worry about accounting for the book's structure in the synopsis. Just tell the reader what happens, in a chronological or other straightforward manner that will allow the reader to follow the plot. For example, if your book progresses on two timelines that converge at the end, you can synopsize what happens during the first timeline, then do the same for the second timeline, then use the last paragraph to explain what happens when the timelines converge. There's no need to jump back and forth between the timelines in your synopsis, even if that's what happens in the book.
If you're using a structure other than straight prose (letters, diary entries, flashbacks, poems, etc.), there's also no need to account for this in the synopsis. Just tell the reader what happens in your story, whether those narrative events take place in prose, poetry, or a different format.
As always, if you're uncertain what method of synopsizing works best for your book, draft a few different synopses and see what works. Have beta readers weigh in. I find chronological is best for easy comprehension of a plot, even a complicated one, but that might not work best for your book. Don't be afraid to try several kinds of synopses until you hit on the one everyone understands. Always remember: the goal of a synopsis is to clarify your plot.
NOTE: It's a good idea to let your reader (especially an agent) know that your book has an unusual structure. However, the query is the place to explain that, not the synopsis. For example, if you have multiple timelines, you can drop a sentence in the housekeeping paragraph of the query that says something like, "BOOK alternates between two timelines, one following Claire through 1950s America and the other following Jamie in the Scottish Highlands of the eighteenth century." (My apologies to OUTLANDER).