How did the Egyptians turn a body into a mummy?

The Egyptians turned a body into a mummy by taking out what rots, drying the rest with salt, and wrapping it in linen. They believed the body had to be preserved so the soul could reunite with it in the afterlife, and the main tool was natron, a natural salt from Wadi Natrun that dehydrates a body.

The first step was to halt decomposition, by removing the internal organs and washing out the body with a mix of spices and palm wine. The only organ left in was the heart, because tradition held that it was the seat of thought and feeling and would be needed in the afterlife. The body was then dried with natron, inside the empty cavity and outside on the skin. The organs were dried too and either sealed in jars or wrapped and put back. Afterwards the body was wrapped in linen bandages, covered with a gum that modern research has shown is both waterproof and antimicrobial.

The whole thing is said to have taken up to seventy days, and it came in three versions, from the most expensive to the cheapest. People who could not afford any of it were believed to be able to reach the afterlife with the right spells.

The part that makes this a good detective story is that nobody wrote down the recipe. No Egyptian text mentions the ingredients, and the Greek and Roman writers only touch on them. So Egyptologists have rebuilt the process from the mummies themselves, from chemistry, and even from an experiment: in 2010 a team mummified Alan Billis, who had agreed to be mummified after a terminal cancer diagnosis, using methods based on 19 years of research.