This KB article outlines the steps on how to map a domain to your site for v8 and v10. Please check your version and here.
You can use any domain name that you own with your site
To map a domain:
1. Click Site settings
2. Click Domain mapping
3. Click Map a domain or click the + button
4. Map your domain using your domain provider's control panel. These control panels differ between providers but the process remains the same:
For the domain you want to map, go to the DNS settings page.
If you want to point the www subdomain to your website (for example www.mydomain.com), add a CNAME record, setting the host to ‘www’ and pointing to XXX
If you want to point the naked domain to your website (for example mydomain.com), add an A record, setting the host to ‘@’ and pointing to XXX
How to map an external domain
5. Click the Test Mapping button to check if the DNS is setup correctly. DNS changes can take anything from two minutes to 24 hours, but typically happen in less than 2 hours
6. When the test passes, click Map domain to create the mapping