After entering the Domain manager and selecting a domain name. Use the lower menu pictured above to add new NS records for your domain. | The Add new NS records screen. Enter a name, and your servers IP address.
After you have created new NS records for your server update your domains DNS servers to use these records. | These simple screenshots should aid the process of registering your server as a name server. If in doubt ask your registrar. Explain you must host your own DNS. They should have help files available for use.