Going to Mikveh on Erev Shabbos:
It is a great Mitzvah to immerse in a Mikveh on Erev Shabbos, and so is the Chabad custom. Immersing before Shabbos draws down the holiness of Shabbos onto one’s soul.  Regarding Erev Yom Tov, even one who is not particular to go to Mikveh on Erev Shabbos is obligated to immerse on Erev Yom Tov.
Preparation for Mikveh: One is to clean his body with hot water and comb through his hair prior to immersion.
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.
Taking a shower after Mikveh: It is permitted to shower after Mikveh, although some are stringent not to do so. Nevertheless, on Erev Shabbos according to all it is better not to do so, in order so one not completely wash off the Mikveh water from his body, as will be explained next.
Drying oneself after Mikveh: Based on the teachings of the Arizal one should not dry the Mikveh water off his body after immersing. Practically we are particular to dry ourselves with a towel after immersion and one who desires to follow the directive of the Arizal is to leave some part of his body not dried. It suffices for one to leave his feet wet in order to fulfill this directive.
Going to Mikveh on Shabbos day: See Volume 2 “The laws of Bathing” Halacha 5 Q&A!
 Ketzos Hashulchan 73 footnote 1
 Immersing before Shabbos is a tradition which was handed down from the Baal Shem Tov.
 Sefer Haminhagim p. 50 [English edition]
 Aruch Hashulchan 260/1
 In order to purify oneself for the holiday. [Mateh Efrayim 625/15]
 This is also required based on the teachings of the Arizal. [Kaf Hachayim 260/4]
 Kanfei Yona 1/95; Mishnes Chassidim Yom Hashishi 7/1; Shlah p. 138a last line in name of Kanfei Yona
 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]
 Sheivet Halevy 7/33; Even regarding a Nida the Rama Yoreh Deah 202/75 rules that only some opinions are stringent and so is the custom. However others argue on Rama and rule doing so is permitted even by a Nida. [See Gr”a ibid; Darkei Teshuvah 202/332; Lechem Vesimla 122]
 Piskeiy Teshuvos 260/1 in name of Sheivet Halevy that so is the custom of the world to avoid doing so despite it being allowed from the letter of the law. See also Tiferes Adam which rules one should never shower after Mikveh.
 Based on Kaf Hachayim 260/5
 As the water is considered holy with the spirit of Shabbos and it is thus proper for the body to absorb it. [ibid] However see Kaf Hachayim 260/8 which writes that this only applies if one did not immerse more than once, otherwise he has already washed away the main Mikveh water. Nevertheless he concludes that even these second waters of the Mikveh has some holiness and is thus not to be dried.
 The holiness of that leftover water from the Mikveh will then subsequently spread to the rest of the body. [Rebbe Shaareiy Halacha Uminhag 1/131]
 Shaareiy Halacha Uminhag 1/131