@nyx @november

https://catholicsbible.com/how-do-you-become-a-nun-in-the-catholic-church/
First Stage: Pick An Order/Community and Express Interest 
Second Stage: Training
Third Stage: Apply for Candidacy
Fourth Stage: Take the First Vows
Final Stage: Take the Final Vows

Nuns are women who take vows to dedicate their life to solemn religious service. So, obviously, the first requirement is that you be female. 
Secondly, you must be a baptized Catholic and part of a church community. 
Many communities will also have a maximum age requirement, usually 40. 

Nuns are centered around convents. They live a life of seclusion and contemplation. 
Sisters on the other hand live within the world and have active apostolic lives, doing acts of service and mercy in their communities.