Writing a synopsis for your novel is never an easy task. Personally, I write my synopsis when I come up with my idea for my novel. You can do it at any point when you're writing your book though. I use the snowflake method by author and writing instructor Randy Ingermanson.
I'll tell you how I came up with the synopsis for my current book. I'm using the working title Souls, which I will change once I get more written. First, I came up with the idea for my story. You can get your idea from anywhere. A dream, a random thought, a fortune cookie - whatever you can find for inspiration. Next, think about who your target audience will be. Will you write for children? For 30-year-old single moms? For 45-year-old men who like woodworking and spy novels?
After you have your target audience, you can think about your sentence. It should be short enough for you to remember so you can tell people when they ask you what your book is about. It should paint a broad picture but give enough detail to be interesting. You want to mention the following: what genre it is; who your main character(s) are; and what your main character(s) want to do.
So, for example: you have an idea that you want to write about a person who wants to heal people with her hugs, except each hug takes 1% off her lifespan. (I got this idea off Pinterest. Great place to search for story ideas - just type in story prompts.) You would first think of your target audience. It might be those 30-year-old single moms. Then you would write out the genre, which might be fantasy. Your main character might be a single mom. She wants to heal people. But there has to be a catch otherwise the story wouldn't be interesting, right?
So your synopsis might sound like this: A fantasy novel where a single mom wants to heal people with hugs but every time she does so it takes months off her lifespan. Now let's make it more interesting. (You should spend about an hour doing this step.) We have to raise the stakes for our main character. Maybe she needs to heal a certain number of people in order to offset a bad deed she did in her youth. Now it might sound like this: A fantasy novel about a single mom who needs to make up for causing a horrific car accident starts to heal people with her hugs while taking months off her life with each hug.
I wrote that synopsis in a very short time; you would take much more time than I did and write it more carefully but you get the point. You have the genre, the main character (you don't use names in this step), the stakes, and the plot. Now you have a killer synopsis and the first step in writing your book.
I hope this was helpful. Feel free to contact me via email if you have any questions.