Script format, some artists only require the elements to be drawn, too deep in text could actually draw far away from the scene than bringing the artist close close, not to mention, is more time consuming to read something in novel format. Storyboards even in stick figure are extremely helpful, doesn't need to be something elaborated but good enough to be understood, while you can provide the key elements about the story, sometimes the artist will have a better idea about paneling, reading flow and how many panels are actually needed in the episode.
It would be great if writers could have clear information of the characters that are going to appear in the story, the specific locations or detailed world building to know the elements to be drawn as well.
You're the one who knows stuff about the characters, artists haven't engineered yet a mind reading ability yet, so of course, background bio, likes, dislikes, physical appearance, height, weight, educations and so many other things are required, especially if you want it to kind of be there in a show-not-tell kinda way. Like a character who has asthma, you'd like maybe a few medicine in their nightstand, other times being agitated and so on, but of course if you're going to use things such as illness the best thing to do is educate yourself on that matter to avoid problems with readers that are actually dealing with those things.
Still... I want to mention that if this is entirely your project, your idea, your vision, your characters, your world, your everything. Then it's probably going to be harder to find an artist to collaborate, and maybe it would be better to hire someone, since that way if you make way too much or too little text but at the same time you're overwhelming to them they won't quit at the lack of creative freedom or to be able to actually put some of their part regarding those aspects.
