First you need to know that all of these :icon___: are icons of deviantART accounts. So for example if you write :iconsteff97:, it will show up like this: .
To create one yourself all you need to do is create another dA account with a 50x50 icon of your emote. Keep in mind though that you can only sign up with the same email (three times ?) I believe.
Since I create so many of these accounts (they are known as PLZ accounts), I use an invalid email addresses. :> Since AOL.com doesn't allow numbers as the first letter of their email, I just use something like "firstname.lastname@example.org". C: PLZ accounts do not have to be verified to be a PLZ account. c: