Though it doesn't really make a difference, I recommend making changes to your Google Sites website first when creating a web mapping for your Site.  You can choose any order you like, but the instructions that follow will go in that order.

Mapping your Site on Google's Servers

1. Navigate to the Site you wish to map
2. Sign in
3. Select "Manage Site" from the "More Actions" drop-down menu
4. Select the "General" tab on the left side of the screen
• If using a stand-alone (non-Apps) Google Site, you will click the "Web Address" tab instead of the "General" tab.
  
5. Scroll down to the "Web Address Mapping" section
• If using a stand-alone (non-Apps) Google Site, you will need to select the "Web Address" tab instead of scrolling down.
  
6. Click the "Map This Site" link
• If using a stand-alone (non-Apps) Google Site, skip this step.
  
7. Type the CNAME (Alias) used in step 5 above into the "Web Address" box.  For this example, we used "www"
• If using a stand-alone (non-Apps) Google Site, you will need to type in your domain name, including the subdomain.  For example:  www.example.com
  
8. "The format of the web address is unsupported."
   
   Be sure to enter www.example.com without the http:// at the beginning to avoid the error message above. 
   
   Also, ensure that no spaces exist before or after the web address.  For example:  " www.example.com" or "www.example.com "
   
   "This location is already in use by another service..."
   
   Google Sites - This Location is Already in Use
   
   Click the "Add" button
   
   
   
   
   

* See the Link Generator for help with locating the administrative pages mentioned above.