There are many great websites out there detailing what is a synopsis (It’s a summary of the story, including key characters, events, and, yes, even the ending.), how to write one that’s compelling, how to organize it, format it, its preferred length, etc, and I’ll link to a few below, but here’s what I learned from writing my latest one: If you keep a separate document throughout your writing process with detailed notes of chapter events, you’ll end with what is essentially a rough outline for your synopsis. That’s not to say that you have to write your synopsis chronologically and with every event listed as a blow-by-blow, but you’ll have a ready template that lays out the storyline.
My document has the working title, date started, chapter titles (with pages listed for easy finding later), and a brief recap of what happened therein. You might think, well, if you wrote the story yourself, you shouldn’t have any trouble remembering what’s in every chapter, but, let me tell you, if it takes you a year to write a story, it’s very easy to forget every detail in a chapter — or forget to tie up that loose end you wrote in as a red herring at the start of the piece. My document helps me regain my thread if I must take time off of writing AND, once I’m finished, to see my path from beginning to end.
Here are links with tips on writing great synopses, realizing that some writers might contradict others. That’s just the way of life, I suppose. Still, these sites are worth checking out: