May one make or undo a hair braid [“Tzama” in Hebrew] on Shabbos?
It is [Rabbinically] forbidden for a woman to braid her hair [or another women’s hair or child’s hair] on Shabbos. It is likewise forbidden for her to undo a braid on Shabbos [even if it was made before Shabbos]. [Some Poskim rule it is even Biblically forbidden to do so. Those women who are lenient to do so are to be protested. This prohibition applies even if one will make a weak braid, and will use a soft brush that does not pull out hair.]
It is forbidden to make a braid or undo a braid on Shabbos.
May one make or undo a braid on a Sheital on Shabbos?
May one simply remove the elastic from the braid, without actually undoing it?
Some write it is permitted to do so, even though it will eventually become undone on its own without the elastic.
 Michaber 303/26
 P”M 303 A”A 20; M”B 303/82; Rambam 20; Chachamim in Mishneh ibid; The following Poskim rule like Chachomim regarding makeup, and the same applies to a braid: Implication of Michaber 303/25 and Admur 320/27; M”A 303/19; Olas Shabbos 303/19; M”B 303/79; See Biur Halacha 303/25 “Mishum Tzoveia”; Kaf Hachaim 303/115-116
 Michaber ibid; Mishneh Shabbos 94b “Hagodeles”
The reason: This is forbidden due to the building prohibition, as it is similar to building and hence the Sages prohibited it. [M”A 303/20; M”B ibid; based on Shabbos 95a] This is supported in the Torah, as the verse states “Vayiven Hashem Elokim Es Hatzela” and the Sages teach that Hashem made a braid for Chava and presented her to Adam Harishon, and thus we see that a braid in the Torah is referred to as building. [Shabbos 95a; Machatzis Hashekel 303/20; M”B 303/82] It is not Biblically forbidden due to building, as the building prohibition only applies to structures. It is also not Biblically forbidden due to weaving, as this prohibition only applies a) To matters that are not attached to a person or the ground, and b) To a weaving that lasts and is permanent. [M”A 303/20; M”B 303/82; Tosafus Shabbos 94b] Regarding why it does not transgress the tying prohibition-see Or Sameach on Rambam 10/8 and Shevet Halevi 1/101
 Michaber ibid; Tosafus Shabbos 57a “Bema”
The reason: This is forbidden due to the destroying prohibition, as it is similar to destroying and hence the Sages prohibited it. [P”M 303 A”A 20; M”B 303/83; Tosafus ibid]
 Rebbe Eliezer in Mishneh ibid, brought in Shaar Hatziyon 303/66; The following Poskim rule like Rebbe Eliezer regarding makeup, and the same applies to a braid: Elya Raba 303/40 in name of Rishonim who rule like Rebbe Eliezer in Mishneh ibid; Yireim, brought in Nishmas Adam; Semag; Ran; Ravan; See Biur Halacha ibid and Kaf Hachaim ibid
Difference between doing it to a friend and to oneself: The Gemara Shabbos 95a differentiates between if a woman makes a braid on her own hair, in which case she is exempt [even according to the approach of Rebbe Eliezer, and only when she makes a braid on her friends hair is she liable, as only then is the braid nice and beautiful.
 The reason: Although it is farfetched to consider a hair braid similar to building a building, nevertheless, the Torah itself testifies that it is considered a category of building, as the verse states “Vayiven Hashem Elokim Es Hatzela” and the Sages teach that Hashem made a braid for Chava and presented her to Adam Harishon, and thus we see that a braid in the Torah is referred to as building. [Shabbos 95a; M”B 303/82]
 Darkei Moshe 303/7; Bedek Habayis; See Beis Yosef 303/26; Kol Bo 31
 M”B 303/82
 The reason: Although doing so does not transgress the building prohibition [as a braid is only considered building when attached to the body, just like Chava], nevertheless, it is forbidden due to the weaving prohibition. Now, although the braid is temporary, it is nevertheless Rabbinically forbidden. [M”B ibid] It is likewise forbidden to undo the braid due to the prohibition of Potzeia, which is undoing the weaving. [Shaar Hatziyon 303/71]
 M”B ibid; See Kol Bo 31, brought in Beis Yosef 303/26
 Chut Shani 36/15, brought in Piskeiy Teshuvos 303 footnote 87