Writing a synopsis is HARD. Because, how are you supposed to take 65-130k words and condense it into a single-page document? Well, the short answer for most is, with a lot of frustration (and a high probability of tears).
Synopses are meant to tell the reader your story--in completion. Yes, that even means the details of the end. While there are many ways to write a synopsis and many resources you can find on the internet, this is the one I've found most success with when writing my own synopses. All I do is answer each question and fill the gaps for transition and flow.
Here's Kaitlyn Johnson's full article, and I suggest reading it in full, but below I've pulled out the questions she uses to get you started.
Beginning: Who is the character? What world do they live in? Why is she the lead of this novel?
Drama: What happens to stir things up? Is a love interested introduced? Does someone get kidnapped or learn of their magical powers?
Next steps: You must include consequences or results of the drama. Whether it be a quest, rebellion, road trip, or family intervention, the reader must know what the main character intends to do about their situation.
Stakes. What is at risk for the main character or those close to them? This is what drives most of the choices in your book, so you better believe the reader needs to know them.
Climax. Where does the height of tension or action occur? What leads up to it and what follows? Keep this section clear and point-blank. Over-excessive explanation can unnecessarily lengthen your synopsis and also confuse the reader as to how the book actually progresses.
Ending. I know, I know, every fiber of your being is fighting against this. My ending isn’t meant to be unveiled before a reader enjoys the whole journey! Guess what? If you jump the shark anywhere in your book, an agent or editor wants to know.
To see how I applied it to my own story, you can see my epic fantasy synopsis here
***And the requisite disclaimer that all writing advice is just advice and never to be taken as the end-all of techniques. This is just one of many ways to write a pitch, and shouldn't be taken as the one-and-only way.***