The synopsis is the document that will sell your book, the cover letter is the opening gambit but the synopsis is the big sell. You need to remember that this is a sales document not (only) an extract of your work. You need to give the publisher/agent an insight into your vision.
Paragraph 1: This is your elevator pitch - you need a few lines to sell your book. This needs to be concise and to the point.
Paragraph 2: This is a more detailed outline of the book. It should consist of a guide to the book's structure and the basic narrative thread of the story. Also include a word count.
Paragraph 3: Here you need to define the genre of your book, detail your potential market, suggest similar titles and give any ideas for future titles.
Paragraph 4: This is about you. Detail any previous writing experience. If you have other books written then include them. Also include an outline showing who you are and why you write.
Rest: This is an extract of your work - normally the first fifty pages. Do avoid the temptation to pick the 'best' bit of your book.
BubbleCow can offer more personalised advice at the BubbleCow website.