Where are you looking to set up shop at? In the main forums, or in a guild? The processes are a bit different depending on where you are as private guilds may have different rules that you need to adhere to.
Generally, what you'll want to do is ensure that you have some samples, know what you want to charge, and know where you want to create the thread. If you're in the main forums, you'll want to go
here.Create the thread. You'll probably want the first post as an introduction, second post for samples and prices, third post for how to order and waiting list, etc- that's just a basic example, make sure you keep a few extra reserve posts in case you ever want to add something.
MAKE IT PRETTY. People are much more likely to buy from shops that look like the owner has put some effort into it.
Bump it. If you don't show it some love once it's open, nobody will ever see it.
Have banners. Link it in your signature, encourage your friends or regulars to do the same. Participate in link exchanges with other reputable shops.
Patience. If it doesn't flourish right away, that doesn't mean no customers will come. People are picky about their art and it's not going to appeal to everyone.
And, as always, never give someone the finished product before they give you the gold. Always show them a heavily watermarked version of the final, accept the payment, then give them the completed clean version of their artwork. Otherwise, they can save it to their hard drive and vanish. Protect yourself.