Your new "reseller" must first register a new domain, or set his existing domain for his name servers. This is done at his registrar's site or if you are also a domain name reseller, and he has registered through you, he can do it in his account. ==> When he/she enters their account, they need to "manage domains" to get to the desired domain name for which to set up the name servers. ==> The area to look for to do this is "Domain Host Summary". (( They might not have this feature in their account features, and their registrar might have a separate sections where to do this, depending on their registrar ))
==> In this "Domain Host Summary" is where they will enter their nameservers, respectively. (( ns1.theirdomain.ext and it's corresponding IP [[ which should be their primary assigned IP ]] & ns2.theirdomain.ext and it's secondary assigned IP ))
==> In the WHM control panel (cPanel) there is an area, under Main >> Resellers >> Reseller's Center >> named "edit privileges/Nameservers" where you will "assign an IP" and "Add an A record for" each nameserver that you setup for your reseller. (( this is at the very bottom of that page ))
That's pretty much it, just follow the same procedure as you did for setting up your own "primary" and "secondary" nameservers for your server.
Hope this helps!