1. Contact fatcow support and ask them to allow you to modify the 'CNAME Alias' or CNAME Record. This is most easily done through live chat (which can be found under the help option in the upper right of the control panel). The change may take a few minutes to take effect.
2. Go to 'Domain Central' under the 'Domain' tab.
3. Click on the domain you want to modify and click on the 'DNS' option.
4. Previously you would have only seen the 'Nameservers' option under this tab. The support team should have changed this so now you have a dropdown list. Go to 'CNAME Alias' in the drop down.
5. Add a new entry. If you would like to redirect to a top level domain type 'www' as the 'Host' and 'ghs.google.com' as the 'Points to'. If you would like it on a subdomain type in 'www.mysubdomain' (where mysubdomain is the subdomain you want)*.
|Here's mine on the subdomain 'blog'|
7. Next go to you're blog's 'Settings' tab. Click publishing. Switch to Custom Domain. Type in your domain (with the www).
That should do it. I know for sure it works on a subdomain, but I haven't tried it on a top level one. Let me know how it works if you try it.
*If you would like subdomain.website.com to redirect to www.subdomain.website.com (as opposed to giving a 404 or redirecting to the main domain) you need to do 1 or 2 of 2 things. I'm not sure if both of these are critical, but I did both, and haven't had any problems. I think the more important one is to go to the '.htaccess Editor' under the 'Website' tab on the control panel. Go to 'URL Redirect' and type 'http://subdomain.website.com' under 'Redirect this URL' and 'http://www.subdomain.website.com' under 'To this URL'. The other thing I did was to repeat the CNAME step (5) except type 'subdomain' into 'host' and 'www.subdomain.website.com' under 'Points to'.