I am using deviant arts own help topic found here http://help.deviantart.com/81/. I will be expanding upon what they have with examples, to make it easier to understand. Just so you know I am using their outline and explanation, if you would like to know which parts were mine and which are theirs just go to the site above
How can I create links to other deviants, deviations, or websites?
There are a few formats you can use, either a link of the users name, displaying the members avatar, or by linking with a personal description.
:iconusername: will display the member's or group's avatar as a link to their page. Remember, however, that the limit for posting blocks of avatars is sixty four per post, and this will not work in signatures.
This seems easy enough, here I will use my own name. :iconamarantheans: will give you :iconamarantheans: