Short answers, pulled from the story.
The opening of the mouth ceremony was performed to restore the fundamental senses of the deceased so they could function in the afterlife. It was believed that without sight, hearing, speech, and the ability to eat and drink, the dead could not navigate the trials of the duat or recite the spells and assertions of innocence required before the gods.
The primary instruments were the ritual adze, shaped like an arm and used to touch the eyes and mouth; the peseshkef, a forked blade made of obsidian, glass, or stone; and a calf's leg, which was touched to the mouth and body to help free the deceased's ba. Incense was also burned throughout the ceremony.
The peseshkef was a forked blade made of obsidian, glass, or stone, believed to mean 'splitter of his ka-spirit.' During the Old Kingdom it was used to sever umbilical cords at birth; its later funerary use carried the same symbolism of crossing a threshold into new life. It was presented to the body or statue as part of episodes 26-41 of the ceremony as recorded in the tomb of Rekhmire.
The ceremony was led by a Sem priest, described as an elderly man of esteemed religious rank responsible for embalming and mummifying the body. During the ritual, the Sem priest took on the role of the god Horus, while the deceased was identified with Osiris. One priest also wore a jackal mask to represent Anubis and kept the corpse upright throughout.
Evidence survives for 75 acts in the ceremony, with the most detailed account coming from the tomb of Rekhmire. This version used a statue rather than a body, and its episodes covered preliminary rites, animation of the statue, meat offerings aligned with Upper and Lower Egypt, a funerary meal, and closing rites.
The ceremony was practiced from the Old Kingdom through the Roman Period, giving it a documented history spanning thousands of years. Evidence for the ritual appears in funerary texts including the Pyramid Texts, coffin texts, and the Book of the Dead.