The easiest way to set this up would be to create an A & NS entry for each nameserver, branded to your domain. We'll use example.com as the domain for the purpose of getting this setup. You'd simply need to change example.com to your domain name.
The IP's listed below are the actual IP's for DigitalOceans nameservers, in order, so you won't need to change them.
First, we'll create/add the A entries:
ns1 » A » 18.104.22.168
ns2 » A » 22.214.171.124
ns3 » A » 126.96.36.199
Next we'll create/add the NS entries:
example.com » NS » ns1.example.com
example.com » NS » ns2.example.com
example.com » NS » ns3.example.com
That'll get you setup and allow you to use the branded nameservers while using DigitalOceans IP's. If you're using a barebones system (no Control Panel), there's a bit more required to get things working (i.e you'll need NSD or BIND to intercept the requests). If you are using a control panel, such as cPanel, DirectAdmin, or Plesk, then you'll be able to add the nameservers and IP's through the GUI.