Setting up private nameservers is fairly simple, however it all depends on where you registered your domain originally.
If PosibleHost did not register your domain or you did not transfer your domain to PosibleHost, you will need to contact the company who registered your domain to setup private nameservers.
Sometimes this can be done via the domain registrar's website.
It's usually in an advanced section and is called 'Register Name Servers' not to be confused with 'Changing Name Servers'
You need to register your own nameservers, as you would register a domain name, just a bit different.
When you get to the screen where to register your nameservers, these are the nameservers you want to register:
ns1.yourdomain.com should be registered to: 18.104.22.168
ns2.yourdomain.com should be registered to: 22.214.171.124
Once you register your nameservers, please log into cPanel and click on the 'Advanced' DNS Editor and add these two A records:
ns1.yourdomain.com should point to : 126.96.36.199
ns2.yourdomain.com should point to : 188.8.131.52
Once you have completed this, you can change your domain over to it's own private/custom nameservers.
If you did register your domain through PosibleHost you can register nameservers through the customer portal by clicking on Domains -> Modify -> Management -> Register Nameservers.