User blog comment:Dogmaster555/Explaine/@comment-3344282-20110208145700

We already have pretty much information about how to become a captain: - if the position is vacant the other captains can give recommendations and the candidate(s) must pass an exam - if you challenge a captain and succeed to kill him in a short time in the presence of 100(?) witnesses, you will become the new captain (like Kenpachi Zaraki did)

I always assumed that a captain can more or less freely choose his lieutenant. As for the seated officers, they earn their rank according to their abilities.