Pick your angle. Decide what it is that you are evaluating whether it's the magazine as a whole, a section, or a series of articles. Thoroughly read and interpret the pieces or issue that you have chosen. Before you can write a concrete piece, you must have knowledge of the subject matter.
Plan the format of your essay ahead. No matter how much you think that you may know about the magazine, you are more likely to write a less than perfect piece if you procrastinate. It's best to plan an essay outline after you finish your research as the information is still fresh in your mind.
Don't be afraid to state your opinion. An evaluation of anything is only complete with an opinion attached to it. However, don't be quick to state opinions without providing reasons for your opinions. If at all possible, list pros and cons of the magazine. This gives your essay readers a chance to draw personal conclusions based on your research. Be careful not to slander the magazine that you are using. Slander is any opinion that is stated without facts to back it up, and is considered unfounded.
Keep essay stylistic expectations in mind. Use proper grammar. Write only in present tense, not in past tense. Cite any quotes that you reference from articles. Also cite the sections that you base your personal opinions around. If you fail to cite these sources, this could be considered plagiarism. Proofread your work.