Whenever your essay topic involves comparison, you can organize in either of two ways. First, you can write about each thing separately and then include a section in which you make comparisons and contrasts between them. With this organization, you would first write about the strengths and weakness of the book, and then about the movie. In a third section you would make a series of statements comparing and contrasting major aspects of them both. If you choose this approach, make your separate discussions of the book and movie parallel — that is, for the movie, address the same points in the same order you used for the book. Also, in the third section of the paper, avoid simply repeating what you said in sections one and two. This approach can be especially effective if the book and movie are drastically different from each other.
A second way of organizing requires you to decide first which aspects of the book and movie you want to compare and contrast (setting, character development, tone, and so on) and then structure your paper around those points. For example, if you use setting, you would have one section of your paper to describe the setting in both the book and the movie and to compare them. Next you compare the character development in both versions, then the tone, then the imagery, and so on. Two advantages of this type of organization are, first, you are forced to focus on specific similarities and differences and less likely to include material that isn't important, and, second, you avoid repetition by ditching the separate compare-and-contrast section.
You can also combine these two types of organization. For example, you may want to discuss the overall plot structure of the movie and the book separately, and then move into a point-by-point comparison of the other aspects of the two versions (characters, setting, tone, and so on).