1) Visit GoDaddy and Login
2) On the "My Account" page, scroll down to the last option called "GoDaddy Auctions" and click Manage
3) You will now be taken to the auctions page. Keep in mind, your account needs to have Auctions added to it, which costs $4.95/year.
4) You might need to login again on the auctions page. Once you do (or if you don't need to login again) you should see tabs towards the top. Choose "Selling List" and then "List a Domain"
From here you'll be able to list it and choose buy it now/offer-counter offer/7 day auction..etc.