It is customary for men to immerse in the Mikveh on Ereb Yom Kippur. This is especially important for the Hazanim who will be leading the prayer services on Yom Kippur, so they can represent the congregation in a state of purity. The work of responsa Min Ha’shamayim (Rabbi Yaakov of Marvege, France, 13th century) mentions that if all Hazanim ensured to immerse in a Mikveh before leading the congregation in Tefila, this would hasten the arrival of Mashiah.
No Beracha is recited over this immersion, as this practice is observed as a custom, and not as a strict Halachic requirement.
If one is unable to immerse in a Mikveh on Ereb Yom Kippur, he can observe this custom by having 9 Kabin – or 12.5 liters – of water poured on him. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that somebody else should pour the water over the person. This quantity of water should be placed into buckets, and just before each bucket is emptied over the person, the next bucket should begin to be poured. The Ben Ish Hai also mentions another option, which is to wash a special Netilat Yadayim forty times with special Kavanot (intentions). Nowadays, when we have showers in the homes, it suffices to stand under the shower until 12.5 liters of waters falls over the body, if one is unable to immerse in a Mikveh.
Strictly speaking, one is not required to remove Hasisot ("obstructions") from his body before this immersion, since, as mentioned, this practice is observed only as a custom, and not as an outright Halachic requirement. This is the ruling of Hacham Ovadia Yosef. Nevertheless, given the importance of entering Yom Kippur in a state of purity, it is proper to remove any rings or other objects from the body, and to cut one’s fingernails, before immersing.
Some have the custom of reciting Vidui (confession) while in the Mikveh. Of course, one must ensure not to utter God’s Name in the Mikveh, but it is permissible to make the Vidui declaration without mentioning God’s Name while in the Mikveh.
The Kavanot that one should have while immersing in the Mikveh on Ereb Yom Kippur are the same as one should have before immersing on Ereb Rosh Hashanah. Namely, the first immersion serves to bring on general purity, and the second is to correct the negative trait of anger. (The Hebrew word "Ka’as" – "anger" –has the numerical value of 150, and when we add to that sum the word itself, we arrive at 151, which is the numerical value of the word "Mikveh.") The third time one immerses in the water, he should have in mind to "sweeten" the harsh judgments through the attribute of divine kindness, and the fourth immersion should be done for the purpose of removing the weekday "garments" from the soul. Finally, the fifth immersion serves to bring on the special spiritual light of Yom Kippur.
Only married ladies that are pure can dip in milveh Ereb kippur. All others like single girls or ladies that are not pure should not dip ereb Kippur. This is from Ner Sion page 118.
It is proper to immerse on Ereb Yom Kippur even if one had immersed on Ereb Rosh Hashanah and had not become Tameh (ritually impure) in the interim.
One should wear fine garments on Yom Kippur. The Gemara (Shabbat 119) infers this requirement from the verse, "Li’kdosh Hashem Mechubad" ("[you should regard] the holy one of God with respect" – Yeshayahu 58:13). "Kedosh Hashem" refers to Yom Kippur, the day that is sacred before God, and we are enjoined to treat it with respect by wearing fine clothing.
The Be’sel Ha’hochma (Rav Betzalel Stern, 1911-1989) ruled that one should not wear on Yom Kippur a white garment with gold embroidery. White symbolizes purity of sin, and gold brings to mind the sin of the golden calf and thus represents sin. It is therefore inappropriate to wear them together. It is permissible to wear gold jewelry – such as rings and watches – on Yom Kippur. However, the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) writes that one should wear on Yom Kippur only jewelry that one normally wears during the week, but not special jewelry that is generally reserved for Shabbat.
Even though no meals are eaten on Yom Kippur, one should place a nice, white tablecloth on his tables in preparation for Yom Kippur in honor of the Yom Tob, just as is normally done for Shabbat. The tablecloths should be left on the tables throughout Yom Kippur. The Ma’amar Mordechai writes that it is proper to adorn the tables in one’s home for Yom Kippur by placing Torah books on them. This is mentioned by the Ben Ish Hai, as well, and Rav Haim Palachi (Turkey, 1788-1869) writes that this is a proper practice to observe.
One should clean the home for Yom Kippur just as one does for Shabbat, because Yom Kippur is called "Shabbat Shabbaton" and should thus be treated with honor just like Shabbat.