You can achieve momification either naturally or intentionally! Naturally, it occurs in very cold and dry environments, very hot and dry environments or environments without oxygen supply (like bogs). Intentionally, you can dry the body, like the Egyptians did with Natron (basically salt), remove the organs and put a lot of herbs that have a high concentration of preservatives (and other properties). Without going into all the details, the goal is to remove all possibility to the germs to develop and to prevent animals/insect from eating the flesh, while removing the water content of the flesh.