There's generally no how per se.
I joined the site and was an active, contributing member of the community, making generally high quality posts, etc. I used the forum frequently, submitted a ton of reports any time I saw anything bad, all while generally not caring if I was made a moderator or not. I was just doing what I generally always did, without a specific end goal in mind.
Eventually there came a time when the existing site staff decided that they needed an extra set of hands. When this happened, the impression that they'd gotten of me through my time here was apparently sufficiently impressive that they decided to offer me the role. I accepted it. There wasn't really anything magical or mysterious about it - it was, in fact, fairly boring overall.
So, now you know how to become a moderator. Let's talk about what happens after you're a moderator.
Congratulations, you get a fancy staff tag. Congratulations, any time there's ever a mess on the forum, you're the person that gets to deal with it. Congratulations, any time cleaning up a mess involves coming down on someone that people like for doing something really bad, you become very (note, I'm being sarcastic) popular. Congratulations, when it comes to dealing with the fallout of those messes, you're the person that has to deal with the after-drama drama.
I'll be blunt. Being a moderator isn't something to eagerly pine after. It's a pain in the ass, and it will invariably shift your perspective of a community by forcing you to always be aware of the worst aspects of it, which often go unnoticed by everyone else due to the actions of the moderation team.