First, the existing moderation team must feel like there is currently a need for new moderators (we currently do). Next, you must be recommended by an existing moderator as a good candidate, and then the rest of the moderation team has a discussion and votes on whether to extend an invitation. It is an invite-only thing :)
As far as what the requirements are: a good candidate is an expert in their field and visits DaniWeb on a regular basis, but even much more importantly, is good at dealing with people, uses excellent judgement, works very well on a team, is mature, does not take things personally, has the same overall direction and goals in line as we do for the future of the community, agrees with the majority of and is able to follow our posting guidelines, and can make impartial decisions.
As far as what the requirements are: a good candidate is an expert in their field
That's not a requirement, though active members who meet the other requirements have a very strong tendency to also be experts who give excellent help rather than receive it. As an example, we were far more likely to single out someone like mike_2000_17 as a potential candidate because he is an expert. He posts very often with confidence and because of that was able to inadvertently show off traits that we look for in moderators.
It is an invite-only thing :)
Kinda sorta. I have it on good authority that Narue originally requested to become a moderator. The usual debates amongst the team ensued, but there wasn't an initial recommendation; Narue started the process by asking, and it turned out well. ;)