Synopsis writing is different than writing a book. I’ve heard many say it’s the hardest thing they’ve ever had to write. I always write mine from a working outline that I make sometime during the novel process. It makes it much easier for me. But, I certainly appreciate any advice I can get on the subject.
I found a great posting for writing a synopsis: How to Write a Synposis. This site had all kinds of goodies, besides that article. So, take some time and investigate.