In order to use a custom nameserver, you must first register it with your domain registrar. Log into your registrar and look for the section that allows you to edit/create nameservers. Once you find that, you will need to refer to your Reseller Account Email which includes the nameserver IP addresses necessary to register these nameservers.
Create your desired nameserver, and apply the correct IP address to each nameserver. Once you have done that, you can then use those nameservers with your domain name. As usual, you will have DNS propagation which can take up to 24 hours.
Make sure that you pay close attention to details; One error with your nameserver or IP address can cause downtime if you don't notice it right away!
