You will need to create the name servers at the place you registered your domain from.

Every registrar has a different process for creating private name servers.

Try and look for something like "register nameserver" "register DNS" "create DNS" or "add DNS" from within your account with your registrar.

The only way to create Name Servers is pointing nameserver to DNS server ip. Default nameservers are provided in welcome email. You can ping the nameservers to obtain ips.

If there isn't a field asking for the ip you are not at the right place.

Private Name Server Notes
Changing your domains DNS is not the same thing as registering name servers. You will not be able to change your domain's DNS to private name servers until your private name servers are registered with your domain registrar. Some registrars don't even have an option for you to create private name servers on our own. In some cases emailing them to do it is the only option.

Help

If you can't figure out how to register your private name servers with your registrar read their FAQ and look for a question along the lines of "How do I register name servers".

If you can't figure it out from either reading their faq or navigating through your account with them you will need to email them asking...

"I want to create name servers for my domain. They are ns1.yourdomainhere.com with the ip of (primary dns server ip) and ns2.yourdomainhere.com with the ip of (secondary dns server ip)"