Login to World of Warcraft and type /qa config to load the settings panel.
In the General settings you set a default for all item groups but I instead like to leave these the way they are and only make sure Smart Canceling is checked.
Next click Profiles in the upper left. Here you can add custom profiles that can be used by any character on your account. I'll call mine Tarou WoW Guides and click Okay or hit 'Enter'.
After that, click Whitelist. If you're like me, you may list on several characters or have friends that you don't wish to undercut. If you enter their name here, the listing system won't undercut them.
Next click Auto Mailer. This basically sets up a player to Auto-mail to. I'll use my alt. I then can click on his name and add any items by typing in their name or selecting a group of items or item below that I want to auto-mail to him. For instance if I add Frost Lotus here, it will automtically mail him Frost Lotus anytime I access a mailbox by checking the auto-mail box at the top of the mail panel. I personally don't usually use this feature but it can come in handy if a player likes to mail certain alts certain items all the time.
Lastly click Item Groups. In this panel every player should pay close attention so you don't accidently list items for strange amounts. Each of these settings will override the general settings. I prefer to use these instead of the general settings so it pretains more to each specific item group better.
For flasks, I like to override stack settings to ignore any stack more than one. This will make sure I don't undercut a stack of twenty or five or whatever. I then set the override post time to 12 hours. Next, I'll set my post cap at fifty but each player should think of what post cap will work best for them on their server. If you have to undercut the competition a lot you'll want a lower post cap like five to ten. I'll then set the per auction to one so it lists in stacks of one and move down to price and set my undercut by one silver. Next, I'll put my bid percent at 95 and set the threshold to 10g. Think of the threshold as the cheapest you are willing to sell the item for. I still make a decent profit at 10g so I'm comfortable with that. I don't worry too much about the next two settings but I do add in a fallback price. This is the price my item will list at if there are none up. Players can be a little greedy here but you don't want to be too ridiculous or noone will buy the item and just think you're a prick of sorts. I'll put mine at 24g 99s which is overpriced but fair.
After that you'll need to add items. You can add items supposedly three ways; by simply clicking on it like I do here with this Eternal Water, typing a portion of or the whole name, or by clicking in the box and shift plus left clicking an item. The last one doesn't work for me so I stick to the first two.
So now that you got all your groups setup it's time to make some items to list.
