How Does A Light Novel Get An Illustrator?

I promise this isn’t just filler, it’s a question that’s been bothering me and this seemed like the best place to reach for an answer.

Just how did VOFAN get attached to Bakemonogatari?

How does an illustrator get attached to a light novel? Do writers go to an artist before they try to get published, or do they meet artists through the publisher? Does the author pay the illustrator before publication, or do both the author and illustrator get paid by the publisher? And if the author does pay the illustrator beforehand, then does the illustrator make any money off of the publication? Is it usually a “professional” illustrator that works on a light novel, or someone the author found on pixiv and launched into stardom with the publication?

Besides my interest as a light novel fan, I’m also interested in this as a writer. I would really love to do some of my works in the style of light novels, so I want to know just how I should go about attaching an illustrator to my works.