First, you'll want your SF admins to attempt to add a Domain. On the Domain Management -> Domains -> Add Domain page, they will see the CNAME information you're looking for in the form of a message that looks like:
Your domain name is a CNAME record that points to
[domain].00da0000000xxxxxxx.live.siteforce.com.
As an example, if you are adding www.example.com,
it must be a CNAME record that points to
www.example.com.00da0000000xxxxxxx.live.siteforce.com.
The 00da0000000xxxxxxx value is unique to your production SF org. Once you have this value, the DNS Admin/Webmaster (you) will need to add a CNAME entry on your side. In this case, it would look something like the following, assuming you don't want to use a subdomain:
www.custom.com.00da0000000xxxxxxx.live.siteforce.com
Once you add the CNAME entry on your side, they'll be able to proceed with adding the custom domain (they'll get an error when attempting to save until SF picks up the entry). In my experience, the CNAME entries can take up to an hour to propagate.
Next, your SF admins will need to add a Custom URL. They'll find this on the Domain Management -> Custom URLs -> New Custom URL page, where they'll select the Domain they just created and the Site it will be associated with (this can be an existing Force.com Site or a Community). You'll most likely want to leave the Path as the default of /.
Full URL rebranding is only available via SSL, and will only work with a signed certificate. Send your SF admins over to the Security Controls -> Certificate and Key Management page and have them create a CA-Signed Certificate with your domain and company's information. You can then use the generated .csr file to purchase a cert from one of the Certificate Authorities. You'll then need your SF admins to upload the signed .cert file to activate the Cert/Key pair.
As a final step, your SF admins will have to return to the Domains page and edit the domain they initially created in order to associate the correct Certificate/Key with it. You may also need to have them head back to the Site/Community in question to correct the Custom URLs in the related list on that page - sometimes when associating a Custom URL with a Site, Salesforce doesn't check the Site Primary Custom URL box next to your Custom URL! They will also want to check the Require Secure Connections (HTTPS) box on the Site/Community, which will force any visitors to your secure URL.
And that's pretty much it, aside from waiting for the DNS entries to propagate. Once they take effect, you should be able to hit your custom URL and be presented with the SF Site/Community Home Page (most likely a VF page).
