How do you put a horse up for auctions?

From the game help:

Members may host their own in-game horse auctions. If you click on the Auctions link under Services, it will take you to a listing of all current auctions in the game. Click on the Auction Home link for your personal auction page. On this page, you can schedule an auction and add your horses to it. Horses may be added or removed up until the Auction starts. Horses that are for sale may not be entered into the auction. Horses may not be sold or abandoned while they are in the auction. Once the auction completes, it will stay in Completed state for two days and then it clears.

On your Auction Home page, you can also see any bids you've made on horses. And you can continue to bid using this page if you like, or go directly to the auction's page to bid. Once the auction completes, the winning bid will stay on your Auction Home page until it clears. To win horses in auction you must have empty stall space or pasture space. If you don't have space, the horse will go to the next highest bidder.