Visit any Auction House and click on the "Game Time" option on the left-hand side of the "Browse" tab.
Instead of a list of items, names, and prices as you are used to on the AH, you will see a screen just displaying WoW Token, the current price (no negotiations), and a Buyout button.
There is a help button at the top right of the AH panel explaining the WoW Token system in more detail.
Click on the Buyout button to obtain WoW Token.
Pick up your WoW Token from the mailbox, as you would with a regular auction.
Click on the token in your bags, which will bring up several confirmation windows to add 30 days of game time to your World of Warcraft account. These windows are handy as they show your current and new account expiration dates.
There is a cap on the amount of pre-purchased game time you can have in advance. On the PTR, we were able to buy and use 10 WoW Token, but then could not buy any more even after learning all the tokens.
If you have a lapsed account and enough gold, you can buy WoW Token directly from the character select screen. Gold will be taken across characters on your selected server. Just click on the "Reactivate with gold" button.
If you have a WoW Token for sale on the AH, you cannot buy one at the same time. The following error message appears: "All WoW Tokens bought from the Shop must be sold before you can buy one from the auction house." This is account-wide, so if character A has a token for sale, character B cannot purchase one.