A reflective essay should describe a realisation. The basic idea is the acquisition of knowledge or wisdom over time, looking back from the present. The structure therefore requires an event, or series of events, in the past, a period of rumination or consideration where the event is analysed, and a conclusion framed in the present that describes what has been learnt. The premise is that the ability to reflect and grow is an important part of our humanity, and our ability to look back honestly and learn from our experience is a component of maturity.
Begin the essay with the event or situation that will lead to the realisation or deeper understanding you wish to present. If you want to precede that with a paragraph that sets up what will come before leaping into the story, that can be helpful and set the tone of the essay. A natural time structure for the essay is to begin with an introduction in the present tense; follow that with the description of the past event described in the past tense; and end with a reflective conclusion in the present tense.
Readers will be analysing not just the substance of the event you describe but how you describe it, so consider the tone of your writing. We make vastly different judgments about what people say depending on their tone of voice, and the same goes for writing regarding choice of words and manner of description.
Develop the essay by adding detail and interesting conflicts that occur along the way, but don't stray too far and keep the end point of the essay in mind. Be sure to include signal events that you can use later in your reflective conclusion. If you want to include thoughts at the time, these can contrast with your later understanding of the events, which demonstrates change and growth.
Once you tell the story or event, and the facts are on the table, conclude the essay with your reflections about what has occurred. Don't just state the differences between what you think now and what you thought at the time, flesh them out with details about why your thoughts have changed. If you can also describe what it is about you that has given you insight, you can also show other qualities of your character, such as curiosity or self-knowledge.