With the majority of the occult action happening off screen, the full extent of the ceremony is unclear, but evidence suggests that part of it was getting Annie to hold a séance with her family so that Charlie’s soul would return to the house. Based on the illustration of King Paimon in the book, it also appears that three decapitated heads are required, as the demon rides with them attached to his saddle. If you’re keeping count, the cult gets these heads from Leigh (presumably willingly), Charlie (who loses her head thanks to the telephone pole), and Annie (who saws her own head off).
Horrible? Most definitely. But “Horrible” and “Horror” have the same root, and in Hereditary what we see is horribleness executed brilliant.