National Geographic has come across the only known footage of an actual human headshrinking ceremony in South America. Warning, the above video might be a bit graphic for some. You don’t see the actual severed head (except in a recreation and in shrunken form), but you see the skin from it boiling in a pot.
Firstly the back of the head would be opened. The skin is sliced free from the skull. Care is taken not to damage the facial features. The skull and remaining flesh is removed. The skin is then boiled in water for half an hour. Any longer and the hair may fall out. After being dried in the sun, the skin is turned inside out.
The process is repeated and can take up to six days, until the head is a quarter of its original size.