First of all, when writing an evaluation essay, the writer must consider for whom this essay is written. If, for instance, if the assignment is for an instructor,the student should be mindful of the tone (formal or informal) that is expected, the amount of evidence required, length, etc.
Essentially, evaluation essays are argumentative. However, the major points that are made are judgments, not opinions. If a student's assignment is the evaluation a work of literature, for instance, in order to form these judgments, it is often helpful to read professional criticisms on these works and, perhaps, then find from these criticisms some characteristics of the work that can be weighed as to their value.
The evaluation essay should address the effectiveness or ineffectiveness of each set of criteria that is used for judgment with examples from the work used to support this judgment. For instance, if the student writes an evaluation essay on Frankenstein as a Gothic novel, then, the criteria becomes the elements of the Gothic style such as setting, atmosphere/style, and characterization as they are addressed and developed in the narrative. For example, Frankenstein clearly has an atmosphere of mystery and suspense, one of the elements of the Gothic novel. With the creation of a malformed being from which Victor Frankenstein runs, there is clearly a threatening feeling and fear of the unknown, along with Victor's overwrought emotions.
Usually, the evaluation essay begins with an introduction of the plot to the reader, focusing on parts of the plot that will lend themselves to evaluation according to the criteria set down in the thesis. Thus, the body of the essay addresses the effectiveness or ineffectiveness of the work in following this criteria in an argumentative style that provides proof.