Read the book carefully. If it is short enough, read it a second time before you start writing the review. If it's a long book, underline key sections you may want to re-read later, during the writing process.
Determine the genre. Is the book poetry? Fiction? A play? Is the author working within the conventions of the genre, or somehow expanding those conventions?
Take copious notes. These should include pertinent quotes you may wish to use in your review and your thoughts in response to the text.
Determine what the author is trying to say. What is the central theme of the book?
Research the author's life online. What other books did she write? What has been the critical response to her books?
Present the findings of your research. Open with interesting facts about the author's biography.
Summarize the plot of the book. If the book is a volume of poetry, give an accurate summation of the themes. Quote frequently to back up your ideas.
Conclude with an opinion. This is your chance to persuade the reader that it is worth his time or to steer him away from the book.