A. The recitation of the Avodah of the Kohen Gadol:
The mentioning of G-d’s name: In times of the Temple, the Kohen Gadol would say G-d’s name in its written form a total of ten times. It was said three times by each of the three confession prayers of the Kohen Gadol, and one time in the lottery made for the scapegoat. When the above parts are recited by the congregation, the name of G-d is not pronounced in its general form (ADO-NOY) as done throughout the year, but rather is simply read as “Hashem”. This is done in order to emphasize that these were the names that the Kohen Gadol would explicitly pronounce. The only exception to this is by the words “Lifneiy Hashem Titharu,” in which Hashem’s name is pronounced regularly as is done throughout the year.
All must recite the Avodah: Even a Torah scholar who desires to spend his time learning rather than recite the extra prayers and supplications, must join the congregation in its recital.
B. Bowing on the floor by the Chazan’s repetition:
Who? The congregation and Chazan all bow when the above words are recited.
The bowing of the Chazan: It is forbidden for the Chazan to move his feet during the repetition and walk backwards in order to perform the bowing. One is to protest against those that do so. The custom therefore is for the Chazan to begin Shemoneh Esrei with enough of a distance from the Amud that he will be able to bow on the ground without moving his feet.
Shabbos Yom Kippur: One bows on the floor by the above words even when Yom Kippur coincides with Shabbos.
How to bow: One places his forehead to the ground [i.e. Nefilas Apayim]. [There are two methods of performing the above bowing: a) The widespread custom is to kneel to the ground, resting his body on his knees and down, and bow his head to the floor until his forehead touches the ground. b) One prostrates himself completely on the ground to the point that his entire body lies flat on the floor. Some write that this later custom should not be seen as a directive for the public. Practically, the widespread custom is like the first method.]
Placing an interval on the ground: It is forbidden for a person to bow his head onto a stone floor, in a way that his head directly touches the stone. This applies even if he is merely kneeling on the ground and is not prostrating his entire body. Thus if the floor in Shul is made of stone it is forbidden to directly touch one’s face to the ground and one is to place an interval between [his head] and the ground. [Any material can serve as a valid interval, whether a towel, tissue, paper, plastic bag, grass and the like. The interval is only required to be placed between the head and the ground and is not required to be placed under the knees or other body parts. If the interval is placed only between the knees and the ground and not between the face and the ground it is invalid.] If the floor is made of wood or other non-stone material, one may bow onto the floor without any interval, having his forehead directly touch the ground, and so is the Chabad custom. However if one fully prostrates himself on the floor with spreading his hands and feet then he requires an interval [between his face and the ground] even if the floor is made of wood or other non-stone materials.
Opening the Aron by Aleinu:
By Aleinu, the Aron is opened until the words “Velo Sam Chelkeinu Bahem”. The Aron is then momentarily closed and reopened by the words Veanachnu Korim.
The custom of the Chabad Rebbeim:
The Rebbe Rashab would prostrate himself completely on the ground, supporting his body only on his fingers and toes. The Rebbe Rayatz told the Rebbe that the bowing is to be done in the following way: One first kneels down on the floor resting his body on his knees and toes. One then spreads his arms and rests on his fingers. One lifts himself off his knees and is hence now being supported only by the fingers of his hands and toes. The Rebbe would close his fist, leaving his thumb out of the fist, and rest on his fist and thumb to support himself.
Are women to bow by Aleinu?
The custom is not to do so. Possibly, this is due to reasons of modesty.
Is the congregation to read Aleinu together with the Chazan?
Yes. The congregation reads along silently together with the Chazan
Q&A on separations
May one use his Tallis to separate between the floor and his head?
If nothing else is available one may use the Tallis that he is wearing to separate between his head and the floor.
May one use his sleeve or other clothing as an interval?
Clothing that a person is wearing is not a valid interval as it is nullified to one’s body.
Is a separation needed by a marble floor?
Is a separation needed by a cement floor?
Is a separation needed by a rubble floor?
A Chassidic message
The Rebbe Maharash noticed a certain Chassid placing a towel on the floor to prevent his silk clothing from becoming dirty. On Motzei Yom Kippur the Rebbe Maharash told him “We speak so much of Bittul but when it comes to actual practice people forget about it”.
C. Praying Musaf prior to a half hour past midday:
It is best not to lengthen Shacharis, in order to be able to say the Musaf prayer prior to 6.5 hours into the day [a half hour past midday]. If the Chazan lengthened Shacharis to the point it is not possible to complete it before this time, then they are to skip Selichos and Avinu Malkeinu in order to pray the Musaf prayer before 6.5 hours.
D. Intention overrides quantity of prayers:
It’s better to say a less amount of Selichos slowly [with proper concentration], than a greater amount quickly [without proper concentration].
E. Ein Kelokeinu:
 Admur 621/7
 The reason: As in Temple times, the Avoda would mainly take place during the sacrifices of the Musaf offerings. [Admur ibid]
 Admur 621/8
 Admur 621/17
 Rama 621/4; Admur 621/12
 M”E 592/1; Machzor ibid
 Alef Hamagen 592/2; Machzor Chabad; Otzer Minhagei Chabad 318
Other customs: Many communities are accustomed that only the Chazan bows with his head to the floor by Aleinu. [Daas Torah 591; Likkutei Mahrich]; Seemingly this is a compromise between the two opinions regarding whether one is to prostrate by Aleinu. [See Otzer ibid]
 621/12; Rama 621/4
 When the time comes for the full prostration within the Avodah recitation, the chazzan may not move backwards to give himself room to prostrate, being that he is still in the midst of the Amidah prayer. Thus the custom is for the Chazan to initially begin the Amidah prayer with a far enough distance to allow him to prostrate.
 Machzor Chabad; See Otzer Minhagei Chabad 323
Other customs: The custom on many communities is not to bow or prostrate during Aleinu on R”H or Yom Kippur that coincides with Shabbos. [Ashel Avraham Butchach 621/4; Hagahos Chochmas Shlomo 592; See Divrei Yatziv 2/265]
 Rama 621/4 and Admur 621/12 [regarding Aleinu of Yom Kippur]
Other Opinions: Some Poskim write that one does not place his forehead to the ground when bowing by Aleinu [of both R”H and Yom Kippur] and this is only done on Yom Kippur by “Kohanim”. [M”A 131/22 “one kneels and bows on R”H by Aleinu however does not place the face to the ground”; M”E 592/1; 621/14; Kaf Hachaim 131/118]
 Alef Lamateh 621/15; Kaf Hachaim 131/115 [“on Yom Kippur one does not prostrate his hands and legs”]; See M”A ibid that says on R”H we are “Korim Umishtachavim” and according to our custom we simply add the touching of the head to the ground. This follows the form of bowing mentioned in Admur 131/1 which is called “bowing to the ground without spreading the hands and legs (even if he sits on the ground and bows his head to the floor)”. And “This was the main form of Nefilas Apayim done in previous times” [ibid]
 See Admur 131/1 that this is called bowing “with spread arms and legs”; This was the custom of the Gr”a [Maaseh Rav 16-The new print] and was the custom of the Chabad Rebbeim. [Otzer Minhagei Chabad 319/321] However see later footnotes that the Rebbeim would support themselves off the ground using their fists and toes and hence perhaps this is not the true prostrating referred to by Admur ibid.
 Piskeiy Teshuvos 591 footnote 11; See Admur 131/1; Rama 131/8
 See Poskim ibid
 Rama 131/8; Admur 131/1; M”B 131/40
The Biblical prohibition-Prostrating: It is Biblically forbidden for one to prostrate himself on a stone floor with spread hands and legs. One who does so receives lashes. This prohibition is learned from the verse “One may not place stone on the ground for purposes of prostrating”. This was only permitted to be done in the Temple.” [Admur ibid] Thus there must be two conditions met for it to be Biblically forbidden: 1) That one is bowing with spread arms and legs. 2) That one is doing so on a stone floor. [M”B ibid]
 This is referred to as Derech “Keida”. [M”B 131/40]
 Admur ibid; M”B ibid; Tur; Beis Yosef
The reason: It is Rabbinically forbidden to bow on a stone floor with one’s face touching the floor even if one does not prostrate himself with spread hands and legs, due to a decree against prostrating in a Biblically forbidden method. [ibid]
 Rivash 412 brought in Beis Yosef; Olas Tamid 131/28; M”A 131/20
 Elya Raba 131/15 in name of Shlah; M”E 621/14; Kaf Hachaim 131/117; See Rambam Avoda Zara 6/7; Piskeiy Teshuvos 131/27
 Rama 131/8 [Place grass between the face and floor]; M”A 131/22 that an interval is valid as rules Rambam. See the following Poskim regarding the two different versions of the Rama here: Taz 131/15; P”M 131 A”A 22; M”B 131/44. See P”M ibid for the reason why an interval is valid.
Does an interval help even when one is prostrating with spread arms and legs? This depends on the two versions of the Rama ibid. Practically we rule like the second version and hence the interval helps, as explained in P”M ibid that when there is a detached interval there is no decree.
 Rama ibid
 Poskim ibid that only the head must have the interval, and that the entire prohibition is placing the head on the floor; See “A Chassidic message” for a story of the Rebbe Maharash on this matter and how one is not to be particular against getting his clothing dirty.
 Pashut from Poskim ibid and so writes Piskeiy Teshuvos 591 footnote 10; 131/27
 M”A 131/20 [that cement is permitted]; Admur ibid [“on a stone floor”]; Sefer Haminhagim p. 120 [English edition]; The Rebbe Rashab would bow on a wooden floor and when he would lift his head dust was visibly seen on his forehead.
Other Customs: Although from the letter of the law one is not required to have an interval between the face and the ground by floor materials other than stone, nevertheless many are accustomed to place an interval on the floor even if the floor is made of wood as a stringency so they don’t come to be lenient by a stone floor. [Toras Chaim 131/9; M”E 621/14; Alef Lamateh 621/15; Kaf Hachaim 131/116; Piskeiy Teshuvos 591/3]
 Rama 131/8; Rivash ibid; Bach 131; Olas Tamid 131/28; M”B 131/40; Kaf Hachaim 131/111
The reason: It is Rabbinically forbidden to prostrate with spread arms and legs on any material, even not of stone, due to a decree against prostrating on a stone floor which is Biblically forbidden. [Levush; Taz 131/14; M”B ibid] Alternatively the reason is because we suspect that there is a stone floor under the wooden material floor and the wood was simply built over it. [Rivash ibid brought in M”A 131/22; Kaf Hachaim 131/114]
The Rebbe’s custom: The Rebbe had a carpet on his Bima which seemingly served as an interval for when he prostrated. However from the memoirs recorded of the Rebbe Rashba, who also prostrated on the floor, it is evident that there was no interval between his face and the ground. Likewise, some Poskim rule that a set carpet is not a valid interval. [Levushei Mordechai 23] Vetzaruch Iyun. Perhaps however one can say that in truth the method used by the Rebbe and Rebbe Rashab is not the real prostration referred to here, as they supported themselves off the ground using their fists and toes and did not fully lie on the ground. Hence it did not require an interval on a non-stone floor. However one who lays flat on the ground without any support is the definition of prostrating that requires an interval on all floors. [Heard from Harav Asher Lemel Hakohen] Vetzaruch Iyun.
 Machzor Chabad; Otzer Minhagei Chabad 318
 Otzer Minhagei Chabad 319-321
 Teshuvos Vehanhagos 2/270; Hiskashrus 947
 M”E 592/1
 Elya Raba 131/15 in name of Shlah; Mateh Efraim 621/14; Ateres Zekeinim 621; Shaareiy Tziyon 131/44; Kaf Hachaim 131/117
 Daas Torah 131; Piskeiy Teshuvos 131/27
 The reason however why a Tallis is valid Bedieved is because it is not attached to one’s body like other clothing. [Shevet Hakehasi 2/61]
 M”A 131/20 records a dispute on this matter.
 Sheivet Halevy 1/23; Piskeiy Teshuvos 131/27
 Sheivet Halevy ibid that when fully prostrating, according to all it is forbidden on all floors, as explained in all Poskim ibid.
 M”A 131/20 [“As cement is not stone”]
 Toras Chaim 131/9; Kaf Hachaim 131/113
 Piskeiy Teshuvos 131/27
 The reason: As it is made from small rocks and cement.
 Rishimos Devarim 2 p. 90 told over by the Chassid Reb Shmuel Gronam Estherman
 Admur 620/1; 286/5
 The reason: The reason for this is because after 6.5 hours into the day, there is question as to which prayer is to first be said; should the Mincha prayer be said first or the Musaf prayer. [286/5]
 Admur 620/2
 Admur 622/1
Other opinions: Some Poskim rule based on Kabala that one is to recite these prayers as usual on Yom Kippur. [Kaf Hachaim 622/6; Piskeiy Teshuvos 622/1]
 Siddur Admur
Ruling of Admur in Shulchan Aruch: In the Shulchan Aruch ibid, Admur rules that the prayer of Ketores is to be recited after Musaf, being that it is not properly mentioned during Musaf in detail. However, in the Siddur, Admur placed both Ein Kelokeinu and the Ketoras at the conclusion of Neilah while after Musaf nothing is said.
 The reason: The reason for this is because it was only instituted after Musaf of Yom Tov or Shabbos when the 18 blessings are not recited. However, on Yom Kippur, in which the 18 blessings are already made mention of in the different Piyutim, it is not needed to be said. [ibid]