The first and most important thing to look at is any information on your instructor's syllabus. This should guide you as to what topic(s) the reflection should be about. Additionally, the instructor should also provide the format -- that is, how long the reflection piece should be.
In regards to giving details on a reflection, a reflection is in essence your thoughts and ideas -- not a summary. Do not use the reflection as a way to reiterate what you have done or read. You can think of a reflection like looking into a mirror and discussing what you would see in the mirror if your own thoughts, beliefs, and ideas would show in that reflection (hence, a reflection paper or reflection journal).
You should still include the basics of writing an essay -- an introduction, usually what your beliefs or expectations were prior to reading the material, having the experience, watching the video, etc. (this is where it depends what the reflection is about). The body of the reflection will be what you learned, what you gained from it, and what you think based on what you read/watched/experienced. Finally, conclude with the overall picture -- what was the main thing you learned or took away from the experience? Has this confirmed what you thought before, or has it changed your views?
Always make sure to proofread any writing assignment before handing it in.