The term "reflective" in "reflective writing" suggests a mirror -- a surface that you look into that allows you to see yourself. Search your memory for an experience that taught you something about yourself. Write out that experience as you remember it, chronologically, with as much detail as possible, with no regard for format or diction. Do not write judgments about the experience or what you think you learned. Simply describe the experience in as much detail as you remember. The experience might be reading a book, learning a skill, breaking up, taking a journey, attending a funeral or anything else that you feel taught you something about yourself.
Go through your chronology, and write about each event in the chronology, this time about what you thought, felt, believed, approved or disapproved when each event happened. Go through the list of your attitudes, thoughts and feelings, and list any ways in which your attitudes, thoughts or feelings have changed since the experience happened. You have now completed your preparatory notes.
Write the piece like a story. It can be a short-short story, a short story or a long story. Begin with a paragraph or two that orients the reader and lets her know who and where you are and what this story is about. After orienting the reader, tell the story; then explain what you learned from the story.
Revise the story to fit the mechanical criteria. Now is when you will ensure you have an introduction, body and conclusion that meet the criteria for the completed assignment. The mechanics will now discipline the writing; but the hard part -- knowing what to write, the actual work of reflection -- is already done. Revise again and again until there is one theme that is consistent throughout the piece, and anything superfluous to that theme is gone.