There are no steps, because it isn't possible--the web GUI needs an IP address, which is a set of numbers. You can, if you care nothing for the security of your data or network, put the web GUI on the public Internet, and give it a public IP address. But even then, the mapping of the domain name to the IP address would be handled by a DNS host, not by FreeNAS.
But, in case it wasn't clear enough above: The web GUI should NEVER be exposed to the public Internet.
Edit: But if exposing the web GUI itself wasn't what you had in mind, maybe you can explain a bit more of what you're hoping to accomplish, rather than just how you think it should be done.