1. The Gemara asks whether the Tevilah before entering the Azarah (Daf 30, #4) must be done with no Chatzitzah.
2. All of the immersions and washings of hands and feet done by the Kohen Gadol were done in a sanctified area, except for one.
3. A person must immerse himself in a Mikvah that contains enough water to cover his entire body.
4. The Mishnah discusses the order of the Avodos performed on Yom Kippur.
5. If the Kohen Gadol was old or particularly sensitive, they would bring hot water for the Mikvah.
A BIT MORE
1. While the usual required immersions must be done without a Chatzitzah (an intervening substance between the body and the water), it is possible that since this immersion is just a "reminder" it is not necessary to check for Chatzitzos. The Gemara concludes that the Rabanan generally instituted their decrees to be similar to Torah law, and therefore the immersion they enacted must be done without a Chatzitzah.
2. The first immersion of the day was done by the Sha'ar ha'Mayim in a non-sanctified area.
3. This amount is calculated as three cubic Amos of water (1x1x3, as the average size of a person is three Amos tall and one Amah wide). This volume translates into 40 Se'ah of water, commonly referred to as the amount of water required for a kosher Mikvah.
4. For example, after his original immersion he dresses in the Bigdei Zahav and then washes his hands and feet. He then slaughters the Korban Tamid (starting the Avodah of the day).
5. The hot water was obviously heated up before Yom Kippur.