Nameservers determine which servers answer for your domain. Changing them is how you point a domain at hosting.
IF THE DOMAIN IS REGISTERED WITH US
Domains > My Domains > click the domain > Nameservers > Use custom nameservers > enter the pair > Change Nameservers.
IF IT IS REGISTERED ELSEWHERE
Log in at that registrar and replace the existing nameservers. The setting is usually called Nameservers, DNS Settings or Manage DNS.
WHICH PAIR TO USE
The pair shown for your hosting account in Services > My Services, and in your welcome email. It is either ns3 and ns4.zillionkinghost.com or ns10 and ns11.zillionkinghost.com, depending on the server your account sits on.
Do not guess, do not copy from an old email, and do not use a pair a friend gave you. The wrong pair leaves the domain unresolved.
REPLACE, DO NOT ADD
Remove the existing nameservers entirely. Leaving the old host's alongside ours produces intermittent, confusing behaviour where the site sometimes loads and sometimes does not.
AFTERWARDS
Allow up to 24 hours, then run AutoSSL in cPanel.
