Following guide explains how to create private nameservers (Child Nameservers) with Register.com.
STEP 1 : Log in to your Register.com account.
STEP 2 : Click on the domain for which you want to create private nameservers.
STEP 3 : Scroll down the page and click on Manage Registered Nameservers.
STEP 4 : Enter the new nameserver and the IP address. Click on the Continue button.
When finished this process, you will have to repeat the process to register your second nameserver.
Note: Remember that after you submit your new name servers and IP addresses to the registry, it can take few hours for the new customers to propagate, after you make the changes.