A reflection essay which, by definition, is a self-reflection essay differs from other types of essays in that it focuses on your feelings and perceptions about a subject that is unique to you. Your instructor's goal or purpose for this essay is most likely to give you the opportunity to speak in the first-person ("I") about a subject or experience that has a deeply personal meaning for you and to see if you can convey your feelings or perceptions in concrete terms to your audience.
Although the subject for reflection essays is usually open to the student, your instructor may have given you several possible topics for the reflection--for example, discussing an experience or person who changed your life in some dramatic manner--and, if this is the case, choose one of these unless you have something significantly better to write about. The essence of a successful reflection essay is a topic that your readers can relate to, and your job is to provide sufficient details about what you think and feel about your topic. I often suggest, for example, that if any students have had what is known as a "coming of age" experience--that is, an experience the changes a child-like perception to a more adult perception--that is a perfect subject for reflection.
A reflection essay first must describe the subject of the reflection, usually, an event or person, but then it becomes an open exploration of what that event or person means to you, not to people in general. Above all else, keep in mind that the essay is intensely personal, and you will be writing in the first person ("I"), which may at first seem uncomfortable because you may have been taught never to use first-person. Your readers, however, are interested not in a general discussion but in your own perceptions and reactions to an experience, and you must be willing to open yourself to the process and discuss aspects of an experience that might be both positive and negative. A reflection essay, then, may conclude on a happy note and, conversely, it may end with a "lessons learned" that is not necessarily upbeat but reflects your perception of the experience or person.
Concrete details in this type of essay are very important. A vague description of the event or person will lead to a reader's vague understanding of how you have reflected on the experience or person you are describing. For every general observation or reflection you make, support it with a highly detailed example that makes the observation vivid and concrete in the reader's mind. Remember always that you and your experience are at the heart of the reflection essay, and only you can adequately convey your own reflection process. If, for example, you tell the reader that an experience troubled you, illustrate what "troubled" means with a concrete, specific description.
Last, keep in mind that reflection essays do not have "right" answers--they describe an experience that is unique to you and, by nature, your reflection may or may not conclude with any definitive statement--in other words, you may always be reflecting on an experience and drawing different conclusions from that experience as you grow older.