Prepare for the exam. If you are warned about the exam beforehand, review pertinent subject material. Don't cram, as the exam is designed as a measure of what you already know, but reacquaint yourself with concepts you have already learned to keep them fresh in your mind. For instance, look over your notes from the last class you took in this subject, or skim your previous textbook. If you are not warned about the exam, free write for a few minutes at the beginning of the allotted exam time to jog your memory on your knowledge and thoughts on the topic.
Examine the exam prompt, and use it to infer what skills or knowledge the instructor is measuring. For instance, if a composition class self-diagnostic test asks you to write a complete essay arguing your position on a current event, the instructor is likely assessing your argument skills and how well you can structure a complete essay.
Develop an outline for writing the test that addresses the skills or knowledge that you noted in step two. Use key words from the test in your major points to ensure you are answering the question completely. Develop your outline by adding sub points that include explanations and examples that both support your major points and exemplify your knowledge of the subject matter. If the exam is timed, give yourself a time limit for each component of your outline to ensure you write a complete exam.
Draft your essay by expanding on your outline. Adhere to the time allotments you set in the previous step. Do not spend too much time perfecting spelling and grammar. Instead, focus on showing your knowledge of the subject matter through writing clear, articulate sentences. If you have time left at the end of the exam, proofread your writing.