The couple documents that specifically describe the mummification method date on the Greco-Roman interval. Many the papyri which have survived only explain the ceremonial rituals involved with embalming, not the particular surgical procedures involved. A text often called The Ritual of Embalming does describe a few of the practical logistics https://elliotjjbmw.bleepblogs.com/35641053/new-step-by-step-map-for-mummification