Mummies are found in the afterlife sections, the earliest of which being Aaru. All you have to do is kill one by burning inflicted by a weapon or the environment like an oil pot. There are a number of mummies located throughout every afterlife section, so the location shown in the video is only for reference.
Here is a video guide with English commentary!
Here is a playlist of guides!
https://www.youtube.com/playlist?list=PL-HeW6XkKdULDFgDhklKO...