Some artists only do digital, posted on line. They usually say in the post if there will be a physical picture or not. Most fics and art are posted online with a note saying it was created for xxx for the yyyy auction. Like this story that Julie just wrote for me.

The way it works, you post an offer at one of the auctions. There is a basic guide. Bidders, reply to your offer. If there is another higher bid, that person replies to the previous bid.
There is a deadline for bidding and close to the end, the mods will put up a clock somewhere on the sight. All three are 'till the last week in January.

After the bidding is over, let the high bidder know that they've won. They will need to make a contribution to a charity and either show you confirmation or the mods. (Sometimes, people make one big donation to cover more than one bid and confirm through the mods.) Once the donation is confirmed, you can create the art!

An alternate plan - In the past, leochi has auctioned popular artwork that she had done recently. Then she has sent that painting to the high bidder.