How do I add my own commentary to mp4 video?

I have a video in mp4 that I'd like to add my own commentary to i.e. add a voiceover over parts of the video, almost like director's commentary on movie's dvd extras. And maybe even be able to add arrows, annotations and the like to frames.

What's the easiest way to do this, using what tools? I'm a video noobie, and haven't used iMovie before—just never been motivated to.

import the movie file to imovie/final cut, then add in your audio layer (commentary), finally export again