Log into the registrar interface.
Once logged in you will see a main screen titled Domain Services in the breadcrumb navigation “My Account > Domain Services”. Scroll down on that page until you see Domain Group Administration which list all of your domain names under “All Domains (x)”. Left click on the title of the domain you'd like to create a name server under.
In the page that loads, you will see a heading titled Global Domain Settings in the right column, click on Register nameservers under that section.
This will load a page titled Nameserver Management. At the bottom of this page under the heading Register a new nameserver; enter the full name of your new nameserver, and the IP of the server that will host it. For example if my domain was example.com and my IPs were 10.0.1.10 and 10.0.2.10, I would register the following: ns1.example.com 10.0.1.10, ns2.example.com 10.0.2.10
Input these one at a time and click the Register Nameserver button below each time.