The title of this post is a little bit of a misnomer because I'm not sure I have the ability to teach you how to master anything. I suspect that only practice can do that.
Margie found a dead body. It was brutal. She cries and carries on, but calls the police. Police come, do their stuff, she's not happy. A few days later or so Margie is back at work when she remembers what the police says. She calls friend. Friend tells her to relax. Yada, yada. After a few days Margie meets a dark and mysterious man who is really creepy....
A synopsis needs to be written with the same skill and effort you put into the entire book. You need to show the editor that you're a hard worker and that at no point will you think anything about your book is a throwaway.
3. Write to the expectations of your genre. In other words, if you are writing a romance, your synopsis should show (notice I said show and not tell) that the reader can expect a romance, romantic tension and probably some romantic scenes. If you're writing a dark mystery than you need to show the darker aspects of the plot as well as how the protagonist will solve the mystery. And the voice should give a feeling of the same darkness your readers will find in the book. If you're writing SF you'll need to show what makes the book SF and not just the general plotting. If you're writing a cozy, show what will appeal to the cozy audience.
4. Make your hook present. Every book has a hook of some sort. That distinguishing factor that makes your book stand out from others in the genre. In a cozy mystery it might be a hat shop, in a romance it might be a Spinster House, in a fantasy it might Steampunk elements or a dystopian world. Whatever it is, you need to make sure you show how this hook factors into the plot and the story as well as showing the plot.
5. Give us everything, but not quite. In other words, a synopsis definitely needs to show us how the book plays out, but we don't need every single tedious thing and every secondary storyline. Stick to the parts that are relevant, but leave some elements open to allow you to edit and play around a bit as you write.
6. Have fun with it. Don't make your synopsis too stiff and formal. Let your voice and your writing shine through. Imagine sitting down to tell someone about your book, or have someone sit down and tell you about your book. What are the important elements and what can the reader discover for herself?