It depends on the order or congregation really. Usually a woman will contact the person in charge of vocations for whatever community she wishes to join. Some visits are made, conversations are had, and an application is made. I’m not sure exactly what the application would be like, as I’ve never applied to be a nun, but I imagine it would not be too different from my seminary application. They’d ask what sort of life the woman has lead, what her prayer is like, why she intends to become a nun, and other questions like that. If she is accepted, she will go through a similar process to what most aspirants to the religious life undergo. First she will undergo a period known as postulancy. The length of this time varies, but I think that a year is probably not an unusual amount of time, but I may be wrong here. A postulant makes no vows, perpetual or temporary. It’s a time to examine whether or not the religious life really is for you, and whether you intend to go forward with this. If a woman discerns during her postulancy that she should stay, she will go on to the novitiate. Here she will make temporary vows of chastity, poverty, and obedience. As a novice she will live as a nun in the community, and thus become acclimated to the life and grow spiritually. After some time (6 years is usual, I think) she will make her final vows and become a nun.