1. The elders of the Beis Din would read to the Kohen Gadol the verses describing the Avodah of Yom Kippur.
2. The Kohen Gadol would be shown the types of animals that would be part of the Avodah on Yom Kippur.
3. The Kohen Gadol was given whatever he wanted to eat during the seven days before Yom Kippur.
4. There is a dispute about whether the Kohen Gadol would be shown a goat before Yom Kippur (see #2).
5. The Gemara discusses specific foods that the Kohen Gadol would not be given even during the seven days before Yom Kippur.
A BIT MORE
1. They would do this in case he forgot some of the details, or in case he never learned them, as was the case in the era of the second Beis ha'Mikdash when people paid to be the Kohen Gadol.
2. This would be done on the morning before Yom Kippur, to further familiarize him with the Avodah.
3. However, the night before Yom Kippur his diet was restricted in its quantity, since too much food would cause him to become sleepy.
4. The Mishnah understands that since the goat is offered to atone for the sins of Bnei Yisrael, seeing it before Yom Kippur would cause the Kohen Gadol anguish. However, there is a Beraisa that states that he indeed would be shown a goat as well.
5. Although the Mishnah implies that he was given any food he wanted during the seven days until nightfall before Yom Kippur, there are various opinions in the Gemara regarding foods that he should not be served even during the seven days.