If you need to edit out faces from a video, especially if the subjects are moving, or you need to add this effect at a specific point in your video, you can use HitFilm Express. The free version of the software has all the functions you need with a relatively easy to use interface. This video will walk you through the process in less than ten minutes. If you want to follow along, please download and install from their website. There are other free tools that can accomplish similar results, but we think this one offers a good user experience and a powerful tool-set for this kind of project for booth PC and Mac users.