The requirements for and what you have to do to become a moderator are the same, participate in the community and be a good human being. You don't have to hit performance criteria, tick boxes or fill in forms.
Potential candidates are usually brought up by a Moderator who thinks they are a good fit for the position, and the rest of the Mods provide their opinions and discussion until a consensus is reached. There is a large human element involved, its not just a matter of having sufficient numbers.
Most I can say is keep answering questions, keep a level head in disputes and be yourself. If someone thinks your a good fit for the job, your name may be raised.