Choose the book you intend to write the book synopsis about. Make sure it is a book you have interest in. If it is one that has been assigned to you, there's no need to search.
Take notes while you read. Taking notes while you read will allow you refer back to specific happenings without rereading the book. Make sure your notes include key characters, events and places.
Weed through your notes and pick out key people and events or places that make the book what it is. Also note themes and plots that run throughout the book.
Write up an outline based on the crucial points you found in your notes. Make sure your outline is organized and detailed enough that you can easily flesh it out and write a very informative synopsis.
Write the first draft of the synopses. As you write it, make sure you accurately describe people, places, and events as they appeared in the book. When writing the synopsis, write it as if you were telling somebody about the book. Give your synopses depth and description to make it engaging.
Read through the synopses and edit as appropriate. Make sure your writing is concise and clear and make sure you accurately followed your outline and notes.
Write up a final draft. Taking into account the edits you made in the first draft, polish up the synopses. The final result should be a fluid piece that represents the book as well as possible.