Acharei Mot - Rishon
G-d spoke to Moshe right after the death of the two sons of Aharon, who brought an unauthorized offering in parshas Shemini (chapter 10). He told Moshe to instruct Aharon not to enter the Holy of Holies except under one scenario: when performing the Yom Kippur service, clad in his linen garments.
Here's the Yom Kippur service: on behalf of the entire community, the Kohein Gadol (High Priest) would take two goats for sin offerings and a ram for a burnt offering. He would first offer a bull as a sin offering to atone for himself and the members of his own household. Next, he would draw lots for the two goats; one would be designated "for G-d" (i.e., for the sacrifice) and the other would be "for Azazel" (which we shall see).
The Kohein Gadol offered a bull as a sin offering on behalf of all kohanim, after which he would bring a fire pan of incense into the Holy of Holies. The smoke of the incense would fill the room. The Kohein Gadol would sprinkle the bull's blood once east and seven times towards the Ark. (On Yom Kippur, during the Musaf service, this is the part that goes, "Achas. Achas v'achas...")
Next, the Kohein Gadol would offer the goat "to G-d" as a sin offering on behalf of the people. He would sprinkle the blood the same way he did the bull's. He would also sprinkle the animals' blood toward the Partition (from the outside). No one else was permitted to be around while the Kohein Gadol performed this service.