A person intending to become a monk would approach the Prior in a monastery.
The prior would ascertain fitness and capacity to live the life of a monk. This involves celibacy and restraining the mind.
When ready, the prospect is admitted and called a postulant.
A prospective monk would then be a postulant for some time, perhaps six months, perhaps longer, one year.
After a time as a postulant, they would be sent to a novitiate, wherein they are trained in prayer, the spiritual life, meditatio, the charism of the order and so forth. At the end of this period of training, the novice takes temporary vows, sometimes called first profession.
The period of temporary vows may last one to ten years. During this time, further spiritual work is undertaken, and perhaps, some tertiary study.
When the postulant's director agrees with the postulant, Final Profession is made, and the postulant is now a monk. Forever.