You should be able to summarize your book in three paragraphs.
Paragraph One: Introduce your protagonist and antagonist; what the inciting incident that starts the story. Give the location, time and what their actions are.
Paragraph Two: What, where, when and why happen in this paragraph. What do they do, where do they do it, when will they do it and why do they do it? Describe actions for both your protagonist and antagonist.
Paragraph Three: Tie the end of the story back to the beginning and the inciting incident. A potential agent or publisher always wants to know how the story will end.
There are many methods used for crafting a synopsis; this is one of them and I hope you find it useful.
All my best,