NB! Your domain must be active in "My Account", in order to create Private Nameservers.
1.1) Click on the link provided:
1.2) Search the Domain name in question.
1.3) Select "Manage"
1.4) Now select "Private Nameservers"
1.5.) Enter both the desired nameserver and its authoritative IP's, and "Save Changes"
*The Nameservers have now been registered the next step would be to remove the old nameserver entries.
2.1) Select "Nameservers", kindly note both your private nameserver as well as the original nameservers are now present.
2.2) Return to the Private Nameserver tab (Step1.4), and remove the original nameservers as follow:
2.3) Return to the Nameserver tab (Step2.1) and verify if the new nameservers are registered.