Preparation for Mikveh-Chatzitza:
It is not required to remove a Chatzitza from the body prior to the immersion of a Baal Keri, and so is the custom. [Nevertheless some Poskim are stringent.] Thus one does not need to cut his nails, clean them, brush his hair or wash other parts of the body. However this only applies if it is found on minority of the body, however a Chatziza on majority of the body is invalid even for a Baal Keri. [Furthermore, some write that initially one is to remove from the body all items that one normally removes upon bathing, such as a watch and clothing as will be explained below.] Despite the above, regarding the immersion of Erev Rosh Hashanah and Erev Yom Kippur [which is done also for the purpose of Teshuvah] one is to remove all Chatzitzas from the body. Thus one is to clean his body with hot water, cut his nails, and comb through his head and beard hair prior to immersion, in order to remove any intervals. One is to cut the nails of both his feet and hands on two different days in preparation for this immersion. One is to also brush and floss his teeth. [See footnote if this should apply also to the immersions of the rest of year, and Erev Shabbos and Yom Tov, for those that immerse for purposes of extra holiness, and not just Baal Keri.]
When on Erev Shabbos is one to go to Mikveh?
One is to only go to Mikveh after midday, or at the very least past the 5th hour of the day. If one cannot immerse at that time then he may immerse anytime in the morning. One is to cut his nails and complete the saying of Shnayim Mikra prior to immersing.
The amount of times one is to immerse:
It suffices to immerse one time for purposes of cleansing oneself from emission of seed [Baal Keri]. One is to immerse three times for purposes of Teshuva. There are numerous opinions mentioned in regards to additional immersions.
 Eshkol [Tefila 1]; Mamar Mordechai  1-2, brought in Shaareiy Teshuvah 88/1; Biur Halacha 88 “Vechen”; Shulchan Melachim in Halacha Lemoshe Dinei Tevilas Ezra 12; Piskeiy Teshuvos 88/8; Leaning opinion of Minchas Chinuch 180/4
Other Opinions: Some Poskim rule that a Chatzitza invalidates the immersion of a Baal Keri. [Amudei Eish Kuntrus Beis Tefila 12; opinion of Gadol brought in Minchas Chinuch 180/4] The Shaareiy Teshuvah ibid leaves this matter in question. The Ashel Avraham Tinyana [Butchach] 88 implies that initially one should remove a Chatzitza [The statement brought in Piskeiy Teshuvos ibid footnote 57 that the Ashel Avraham holds it does not even contain a Midas Chassidus is inaccurate] The Rashba [Yevamos 47] rules that a Chatzitza invalidates the immersion of a Baal Keri. Nevertheless this only refers to one who needs to immerse Biblically, such as in the times of the Temple. However for the immersion of Takanas Ezra even the Rashba agrees it is not necessary. [Shulchan Melachim ibid]
 See previous footnote “Other Poskim”
 Piskeiy Teshuvos 88/8
 Biur Halacha 88 “Vechin”; Halacha Lemoshe Dinei Tevilas Ezra 12; as is implied from the Eshkol
 Piskeiy Teshuvos 88/8; Vetzaruch Iyun on his source as the simple reading of the Poskim ibid do not imply that a differentiation exists between forms of Chatzitzas.
 Ben Ish Chaiy Netzavim 3; Kaf Hachaim 581/84; This is also required based on the teachings of the Arizal. [Kaf Hachaim 260/4]; M”E 606/8 regarding Erev Yom Kippur
 M”E 606/8 regarding Erev Yom Kippur
 The above statement of the Poskim that validate the immersion of a man even if he has a Chatzitza refer specifically to one who is immersing do to being a Baal Keri. However when immersing for Teshuvah purposes, as is done on Erev Yom Kippur, the Poskim ibid write that one is to remove all Chatzitzas. This arouses a question regarding those that are accustomed to immerse daily, irrelevant of being a Baal Keri, due to extra purity. Perhaps they too should be careful with all the laws of Chatzitza. Practically, the custom of Chassidim is to shower prior to Mikveh, although they are not particular to cut the nails and brush the knots out of the hair during this time. Vetzaruch Iyun.
 Kanfei Yona 1/95; Mishnes Chassidim Yom Hashishi 7/1; Shlah p. 138a last line in name of Kanfei Yona
 Mateh Efrayim 625/14, in name of Shaar Hakavanos and Peri Eitz Chaim; Chayeh Adam 138/5; Kaf Hachaim 581/82
The reason: As from that time and on the radiance of Shabbos begins to shine. [Mateh Efrayim 625/14, in name of Shaar Hakavanos and Peri Eitz Chaim]
 Mateh Efrayim 625/14
 See Kaf Hachayim 260/1 and Mate Efrayim 625/13; Shlah 138 that the nails are to be cut prior to Mikveh.
 M”A 285/1; This was the custom of the Arizal [Brought in Kaf Hachayim 260/7; M”A ibid] and so writes Shalah p. 138 explicitly in Hagah that one is to read Shnayim Mikra prior to immersing. So rules also Oar Tzadikim 28/18 stating that if one immerses prior to Shnayim Mikra he does not have the ability to receive the holiness of Shabbos.
This is unlike what is written in the Peri Megadim 260 A”A 1 in name of Eliyah Raba [260/4] in name of Shalah that one is to immerse prior to Shnayim Mikra. After researching this seeming contradiction the following was discovered: The Eliyah Raba [260/4] himself never makes such a claim and rather simply states in name of Shalah that one is to cut his nails prior to Shnayim Mikra, and does not discuss immersion in that regard. Furthermore, in some prints of the Peri Megadim the entire novelty of immersion after Shnayim Mikra was placed in brackets hence lending suspicion as to the accuracy of what in truth the Peri Megadim wrote. Due to all above seemingly there was a misprint in some versions of the Eliyah Raba or the Peri Megadim. This seemingly occurred due to a misreading of the word “Yitol” [take the nails] which was read “Yitvol” [immerse] and hence this caused the change. In any event the ruling of the Shalah is clear as written in his Sefer Shlah Hakadosh, that one is to immerse after Shnayim Mikra! In light of all the above there is no room for the ruling brought in Piskeiy Teshuvos 260/1 or 285/1. [In the new edition of Piskeiy Teshuvos 260/4 they fixed the ruling and wrote as we stated above.]
Other Opinions: Siddur Yaavetz rules one is to immerse prior to Shnayim Mikra.
 Shaareiy Halacha Uminhag 1/131
 Admur 606/11-12
 2, 4, 5, 13, 14, 39 [Shalah]. The Arizal would immerse 2 times on Erev Shabbos, once to remove the mundane garments of his soul, and the second to attain the holy garments of Shabbos. If one is a Baal Keri then he is to dip three times in total according to the Arizal. [Kaf Hachayim 260/6]
 The Rebbe concludes “Who am I to arbitrate between all these opinions”. [ibid]