It's easy. On your profile bar (where it says "Welcome") there is an option that says "Settings", click it, and scroll down. There should be an option that says "Edit Profile" click that one and there, you'll see the option to make a user title. Write one and click on "Save settings" at the bottom.
It will appear under your username (seriously, nobody thought of using that name before you?) in every post you make and in your profile.
Oh hey! Here's a random person commenting on your page. That's amazing that you were able to get that username....scrolling down your page I can see a lot of people asking you how you got it, so I won't bother you by repeating the question again~ ^^
I'd say you're pretty lucky to have gotten it though....Link!