There's a couple options available to use a domain name you already own. First, let's take a look at using Nameservers to point your domain:
1. Sign in to your Zillow profile and click on My Website
2. Click on Domain/URL
3. Type the domain you own in the box labeled What domain do you want to use?
4. Click on I Own This Domain
5. Click Confirm Domain Change after carefully reading this screen
6. Copy the 4 unique Nameservers assigned to you. They'll look something like this:
Name Server 1: ns-1234.awsdns-56.org
Name Server 2: ns-789.awsdns-10.com
Name Server 3: ns-123.awsdns-45.net
Name Server 4: ns-6789.awsdns-10.co.uk
7. Save these unique Nameservers in your domain registrar account and you're done!