How should a woman cut her hair in Hajj and ‘Umrah?
Question

After my mother performed ‘Umrah, she cut one lock of her hair, as she didn’t know the ruling. What is the ruling on that?
Praise be to Allah.Shaving the head or cutting the hair is one of the obligatory acts of ‘Umrah. Women do not have to shave their heads; rather what is prescribed for them is cutting the hair. It is essential to shorten all of the hair, according to the correct opinion. This is the view of the Maalikis and Hanbalis. If she has braids, she should remove a little from the end of each braid. Otherwise she should gather her hair and cut something from the ends of all of it. What is recommended (mustahabb) is to remove a fingertip’s length, but less than that may be removed, because no specific amount is mentioned in the texts. 
Al-Baaji (may Allah have mercy on him) said in al-Muntaqa (3/29): With regard to women, when a woman wants to enter ihram, she may braid her hair, then when she exits ihram she may cut something from the ends of her braids. 
How much should she cut off? It was narrated from Ibn ‘Umar that he said: The length of a fingertip. Ibn Habeeb narrated from Maalik that it should be the length of a fingertip or a little more or a little less. Maalik said: In our view there is no specific amount, and whatever she cuts off is acceptable, but it is essential to cut something from all of the hair, whether it is long or short. End quote. 
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (3/196): It is obligatory to cut or shave all of the hair; this (with regard to cutting the hair) also applies to women. This is our view and is also the view of Maalik. End quote. 
He also said: Any amount that is cut from it is acceptable. Ahmad said: He should cut off a fingertip’s length. This is also the view of Ibn ‘Umar, ash-Shaafa‘i, Ishaaq, and Abu Thawr. This may be understood as meaning that it is mustahabb, because of the words of Ibn ‘Umar. End quote. 
He also said (3/226): A woman should cut a fingertip’s length from her hair. What is meant by the fingertip is the end of the finger, from the highest knuckle. What is prescribed for women is to cut the hair, not to shave the head, and there is no difference of opinion concerning that. Ibn al-Mundhir said: The scholars are unanimously agreed on this. It was narrated that Ibn ‘Abbaas (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Women do not have to shave their heads; rather women have to cut their hair.” Narrated by Abu Dawood. It was narrated that ‘Ali (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) told women not to shave their heads. Narrated by at-Tirmidhi. Ahmad used to say: She should cut a fingertip’s length from each lock of hair. This is the view of Ibn ‘Umar, ash-Shaafa‘i, Ishaaq and Abu Thawr. Abu Dawood said: I heard Ahmad being asked whether a woman should cut something from all of her hair. He said: Yes; she should gather her hair at the front of her head then remove a fingertip’s length from the ends of her hair. End quote. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in ash-Sharh al-Mumti‘ (7/329): “A woman should cut a fingertip’s length from her hair” refers to the top of the finger. In other words, a woman should take hold of her braids, if she has braids, or the ends of her hair, if she does not have braids, and trim the length of a fingertip from it. That is approximately two centimeters. With regard to what is common among women, which is wrapping the end of the hair around the finger then cutting it , that is not correct. End quote. 
Based on that, a woman who cut only one lock of her hair has not cut it in the manner required. What she must do now is cut her hair in the manner we have described, but there is no penalty on her for what she may have done previously of things that are forbidden in ihram. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said concerning a woman who had not completed her ‘Umrah: With regard to what she may have done of things that are forbidden in ihram, if we assume that her husband had intercourse with her – and intercourse when in ihram is the most serious of the things that are forbidden – there is no penalty on her, because she was unaware (of the rulings), and anyone who does any action that is forbidden when in ihram out of ignorance or forgetfulness, or because he was compelled to do it, does not have to pay any penalty."(Majmoo‘ Fatawa Ibn ‘Uthaymeen 21/351). 
He (may Allah have mercy on him) was also asked: A man cut his hair on one side after ‘Umrah, then he went back to his family and he found out that what he had done was not correct. What should he do? He replied: If he did this thing out of ignorance, than what he must do is take off his ordinary clothes now (and put on his ihram garments), and shave his head completely or cut his hair. What he did is pardoned, because he did not know. Shaving or cutting the hair does not have to be done in Makkah; rather it may be done there or elsewhere. But if what he did was based on a fatwa from one of the scholars, he does not have to do anything, because Allah says (interpretation of the meaning): “Ask the people of knowledge, if you do not know” [al-Anbiya’ 21:7]. And some of the scholars said that cutting part of the hair is like cutting all of the hair."(Al-Liqa’ ash-Shahri, no. 10 )
Women do not have to change their clothes before cutting the hair, because it is not haraam for them to wear ordinary clothes when in ihram; rather they are only forbidden to wear the niqab and gloves. 
And Allah knows best.