This comes up a lot. The cookie cutter method works well if what you want to mask is similar to one of the presets. I goofed yesterday thinking it had a rectangle, it doesn't. but it does have a circle and oval which works decent to block out someone's face like you see on COPS and other shows.

Simple...

1. duplicate your video track and align vertically.
2. Drop cookie cutter on top track, adjust mask, add blur filter
3. use key frames to track.

If you really want to get precise, you'll need to step through the frames and adjust the size/position of the mask controlled by the cookie cutter filter setting key frames as you go. Not hard, but can take time if you have a long run to do.