Exam: make one, talking the whole time, helping us see everything you see about how the clip works, using every film term you can muster, but also noting foreshadowing, themes, closures, colors, rhythms, etc. (You might go back and review what you know about film conventions.) Your overall goal here is not to entertain us by your personality — it is to help us see and understand things we would not unless we have been enlightened by your insight.
Take care to make your voiceover clear and easy to hear and understand and use ducking to keep a little of the actual soundtrack audible.
Sign up here — pick a five minute block, watch it many times until you find a one-minute block within it that will let you show off best, most observant, insightful, and film-informed self. Clip the one minute block out, using MPEG Streamclip if you want and haven’t updated to Mavericks yet, or using QuickTime Player’s trim function if you have updated or if find it easier. Then slam it into iMovie and add your voiceover.