I show you step by step how to do it in my book, if you're interested.
If you understand how to use the Track Matte, it's not that hard. You just put your video on Video 1 and a duplicate of it on Video 2. Then you blur the video on Video 2 and use the Track Matte to limit the blurred video to a small area of the video, which you keyframe to follow the face around.
You can also use the Effects Mask or even a Garbage Matte to similar effect.