There is three major reasons to write reviews. First, a review requirement ensures that students will do the assigned reading, or whatever else is being reviewed. It is much harder to fake familiarity with a work when one is required to write about it. Second, reviews are logistically easy. Students assigned a research paper will necessarily spend a lot of time hunting for a topic, finding sources, and wandering down dead ends. In contrast, if an instructor assigns a review of required reading, the students begin with a topic and their sources, so they can spend their time reading, thinking, and writing. Third, and by far most important, review essays provide practice in one of the most valuable skills offered by a liberal arts education: the skill of critical reading. When I assign a review, it is this skill that I hope to see displayed.
When starting to write a review, think about what you're writing about and describe it. Write a summary as short as possible without leaving ANYTHING out.
The second step is to write the review itself