Errr.. it's not really that difficult.
Just setup your domain name(s): Domains - Manage Domains - [Add New Domain / Sub-Domain]
Once that's established, next to your domain name you should see a click-able "DNS" link. Click on that, scroll down and enter in the given IP's into your external DNS provider's records.
The only real downside to this, as I do the same thing, is that we don't have static IP's and therefore you'll either need to re-modify the IP's when/if they change, and/or you could write a script to monitor changes in the IP addresses and modify them for you.