This is a simple concept to understand! The person posts a link to their character that they would like somebody to draw, somebody else claims it, and then THEY post a link to a ref of their character. When you've claimed a character to draw, you MUST draw them and send the person a link (preferably by editing your comment) before you claim another character to draw. You can't make a mass-claim collection. Also, you can't not do the drawing. 7 days and then you'll be PM'd by me or the person. Any longer and you'll get constant PM's about it.
dA is preferred, purely because of the Sta.sh where you can put the image without needing a description, and then you can remove it without watchers seeing it. People WILL be allowed to add the drawings to their characters gallery, but ONLY if they credit the person who did the art.
You can ask for art of ANY character, it doesn't need to be a Marvel FC, but don't always ask for the same character as people will eventually stop doing art for them. Make sure it's a character you are about and won't eventually sell, either, unless you are willing to share some of the cost with who drew art for them. It's not fair if you get people to do art of a character just so you can sell them for more.
Also, you must care about the art you do, and don't half-ass it at all. That's not fair on the person you did the art for, either.
I'll start (of course)! I'm always looking for art of one of my current favourite characters, Abbey! She has the most art out of all of my characters so far, but I'm always looking for more. She should also be quite easy to do, since her markings are interchangeable and aren't set to a certain place. The harsest should be her cheek fur, which goes out, but even then that isn't always set how it can be done and has changed a lot with each artist's style.