The Greek historian Herodotus on the process of mummification -- and he has been proven accurate:

The mode of embalming, according to the most perfect process, is the following:- They take first a crooked piece of iron, and with it draw out the brain through the nostrils, thus getting rid of a portion, while the skull is cleared of the rest by rinsing with drugs; next they make a cut along the flank with a sharp Ethiopian stone, and take out the whole contents of the abdomen, which they then cleanse, washing it thoroughly with palm wine, and again frequently with an infusion of pounded aromatics. After this they fill the cavity with the purest bruised myrrh, with cassia, and every other sort of spicery except frankincense, and sew up the opening. Then the body is placed in natrum for seventy days, and covered entirely over. After the expiration of that space of time, which must not be exceeded, the body is washed, and wrapped round, from head to foot, with bandages of fine linen cloth, smeared over with gum, which is used generally by the Egyptians in the place of glue, and in this state it is given back to the relations, who enclose it in a wooden case which they have had made for the purpose, shaped into the figure of a man. Then fastening the case, they place it in a sepulchral chamber, upright against the wall. Such is the most costly way of embalming the dead.

If persons wish to avoid expense, and choose the second process, the following is the method pursued:- Syringes are filled with oil made from the cedar-tree, which is then, without any incision or disembowelling, injected into the abdomen. The passage by which it might be likely to return is stopped, and the body laid in natrum the prescribed number of days. At the end of the time the cedar-oil is allowed to make its escape; and such is its power that it brings with it the whole stomach and intestines in a liquid state. The natrum meanwhile has dissolved the flesh, and so nothing is left of the dead body but the skin and the bones. It is returned in this condition to the relatives, without any further trouble being bestowed upon it.

The third method of embalming, which is practised in the case of the poorer classes, is to clear out the intestines with a clyster, and let the body lie in natrum the seventy days, after which it is at once given to those who come to fetch it away.

Source: Herodotus, The History Book 2, chapters 86-89