One thing you will need to do in this sort of evaluation essay is to explain to your audience the content of what you are evaluating, because they won't necessarily have read it. So the first thing I would do is to summarize the opinion article. However, you may need to give the reader some context for that opinion before you summarize if they are not familiar with the issue. I generally tell students to start with something that grabs the attention of the reader. For example, if the issue is about a bad law, you could start your paper with a story about someone encountering that bad law and all the problems that makes them have. Then you can explain the opinion article and what they think about that law. Your thesis sentence would be something like: "Does Ms. Johnson write effectively to convince her audience that....." Your article will evaluate the person's analysis of the situation as well as whether they have effectively argued their point of view. Your evaluation can consider the tone of the article, the use of examples, the quality of the evidence, the effectiveness of the arguments presented and whether or not their ideas offer the best way of looking at this situation.