Khula is done by asking the husband to let you go from his Nikah by giving you Khula. Normally Khula is resorted to because the husband is unwilling to give Talaq. In order to make him agree the wife offers him, either to forgive any Mahr he owes her or makes an offer to give an agreeable sum of money. She makes this offer by saying:
"leave me for X amount," or "Do Khula for X amount."

The husband then merely says: "I accept" or "I give you Khula" and this will be considered a single irrevocable Talaq (One Talaq Bain).

It is also possible that no money or property be made the counter-value for the Khula. If the cause of separation is the ill treatment of the husband, it is disliked for him to seek more than the dowry. It is also possible that the man initiate the process by making the offer. Whatever form Khula takes, one will have noted, the agreement of the husband is necessary.

The alternative to Khula, is simply to ask the man to give Talaq, but in this case any rights upon him will still remain obligatory upon him, in terms of outstanding Mahr.

There are no particular reasons for which Khula is available to the woman.
However, it is obvious that the Shariah considers it highly disliked and blameworthy to break a marriage without a reasonable cause.