The Confession Prayer:
The obligation: It is an obligation to say the confession prayer on Yom Kippur being that it is a day of atonement and forgiveness, and whenever we receive atonement, a confession prayer is said. [Likewise, there is an obligation to perform Teshuvah on Yom Kippur, and whenever one performs Teshuvah he is Biblically obligated to confess.]
When is it said-Mincha Erev Yom Kippur and as Yom Kippur begins? From the letter of the law, the confession is to be said from the start of the atonement, meaning from the start of Yom Kippur, which is by nightfall. However, the Sages instituted for the confession to also be said before Yom Kippur, by Mincha. Accordingly, it is to be said by Mincha due to the Rabbinical institution, and is also to be said by Maariv, as is required from the letter of the law. However, some Poskim rule that from the letter of the law the confession is to be said bef ore Yom Kippur begins, which is as it becomes dark, and not after Yom Kippur begins after it becomes dark. Nonetheless, also in their opinion the Sages instituted for the confession to also be said by Mincha. According to this opinion, the confession must be said by Mincha due to the Rabbinical institution, and is also to be said right before Yom Kippur as is required from the letter of the law. Practically, although we rule like the first opinion, nevertheless it is proper for every person to be stringent upon himself like the latter opinion. Therefore, in addition to the confession prayer said after Mincha of Erev Yom Kippur and at Maariv of Yom Kippur, one is to also confess upon the approach of Bein Hashmashos. [Practically, being that Kol Nidrei is said at this time, one should say this confession prior to Kol Nidrei.]
Confessing in every Shemoneh Esrei of Yom Kippur: In addition to the above confessions, the sages instituted the confession prayer to be said after [after Sim Shalom of] each Shemoneh Esrei prayer which is recited on Yom Kippur. [This is with exception to the prayer of Neilah, in which only Ashamnu is recited and not the full confession liturgy.]
The confession within the Chazans repetition: The Chazan recites the confession prayer during Chazaras Hashatz. It is said in the midst of Shemoneh Esrei, as opposed to after its conclusion. When the Chazan says the confession prayer, the congregation says the prayer together with him. They are not obligated to mention their particular sins in this confession, and rather simply saying “We sinned against You” suffices. [Nevertheless, the custom is to repeat the entire confession.] It is forbidden to speak while the Chazan says the confession.
What is one to say in his confession? One is not obligated to mention specific sins during his confession, and even saying “I have sinned” suffices. Nevertheless, it is initially proper to mention one’s particular sins during the confession. One’s particular sins are to be mentioned quietly. One may mention sins from previous years which he already repented on, as one should view his sins as if they are constantly before him.
How to say the confession: One is to stand while reciting the confession prayer. He is not to lean on an item in a way that if the item were to be removed it would cause him to lose his balance and fall. It is proper to bend one’s back upon saying the confession, in order to say it with great humility. Upon confessing, one is to hit his heart [each time he recites a particular sin]. Others are accustomed to hit their chest.
How to achieve atonement: The day of Yom Kippur only atones for the sins of a person who believes in the specialty of the day of Yom Kippur, and repents.
One recites the confession prayer after Sim Shalom of each Amidah prayer of Yom Kippur [with exception to Neilah in which it is not said]. One also recites the confession prayer at Mincha of Erev Yom Kippur. Upon the approach of dusk of Yom Kippur, one is to say the confession prayer.
May an old or weak person lean while reciting the confession prayer?
Some Poskim rule it is permitted for them to do so.
Is one to mention his personal sins within the confession prayer printed in the Siddur?
It is proper for one to silently mention any of his personal sins during the confession prayer recited. [Nevertheless, it is to be recited only after he has completed the regular Nussach of confessions.]
What confession prayer is to be recited by Bein Hashmashos, before Kol Nidrei?
What is one to do if he forgot to recite the confession prayer in Shemoneh Esrei?
If one forgot to say it during Shemoneh Esrei, he may say it after Shemoneh Esrei.
If one made an invalidating mistake that requires repetition of Shemoneh Esrei, must he also repeat the confession prayer?
 Admur 607/1-5; See Sefer Hachinuch Mitzvah 364
 Admur 607/1; See Sefer Hachinuch Mitzvah 364 who explains the confession is part of the Mitzvah of Teshuvah which is required on Yom Kippur, and one who does not confess, transgresses this positive command
 This is evident from the confession prayer said over a sacrifice that was brought for atonement. [ibid]
 Sefer Hachinuch Mitzvah 364; See Shaareiy Hamoadim p. 161-168; Likkutei Sichos 29/203
 Michaber 607/1; Tur 607; Rosh 8/19; Yuma 87b
 The reason: This is due to suspicion that one may consume too much alcohol during the feast of Erev Yom Kippur, and thus forget to confess. [Admur ibid]
 Ramban Yuma ibid
 The reason: This is so, in order for he enters the day free of sin. [Admur ibid]
 The reason: Due to suspicion that one may consume too much alcohol during the feast of Erev Yom Kippur, and thus forget to confess right before Yom Kippur, therefore the sages instituted for the confession to be said by Mincha of Erev Yom Kippur. [Admur ibid]
 Admur ibid; Beis Yosef that so is custom
 Admur ibid; M”A 607/7; Shlah Yuma Teshuvah p. 228; Aruch Hashulchan 607/2; See Nitei Gavriel 27 footnote 14; See Keser Shem Tov 330
 Begins about 4 minutes after sunset
 Admur 607/1
 The reason: This was instituted to be done just in case one sinned after the previous confession. [Admur ibid]
 In the Siddur of Admur, the detailed confession is omitted and one rather only says “Ashamnu Bagadnu.”
Ruling of Admur in Shulchan Aruch: In the ruling in Shulchan Aruch 607/1, Admur explicitly rules that the confession prayer is to be said also by Neilah, without differentiation.
 Admur 607/8
 Admur 607/4
Other opinions: See Sefer Hachinuch Mitzvah 364 who explains that one must confess the particular sin, ask for atonement, and state that he will not repeat the sin.
 The reason: As through doing so one comes to a greater level of remorse and regret. [Admur ibid]
 The reason: As it is forbidden to publicize one’s sins to the public. [Admur ibid]
 Admur 607/9
 Admur 607/7
 Admur 607/16
 Birkeiy Yosef 131; however see Piskeiy Teshuvos 607/4 in name of Peri Megadim
 M”E 607/8; Chayeh Adam 143
 See M”E 607/12 that one should not change or rearrange the order of Viduiy recited in the Siddur.
 M”E 619/8
 See Piskeiy Teshuvos 607/2
 Meishiv Halacha 237; Chochmas Shlomo 623 leaves this matter in question; Shevach Hamoadim Yom Kippur 16; Piskeiy Teshuvos 607/1 in name of Rav SZ”A
 Sheivet Halevy 6/73; Piskeiy Teshuvos 607/1 in name of Rav SZ”A