If you're looking to Change the Domain for a site, click here for instructions.
Here's how, using Williamsprobuilderdomain.org as our example:
(Note: the instructions are very similar for GoDaddy as well and other companies)
Let the client purchase the domain name from a williamsprobuilderdomain.org. This can be done for about $10.00 first year and every other year it would be $17/year.
Log in to your account and go "My Products" then look for the section titled "Domains" and click the "Manage" button next to it.
From the "My Domains" page, find the domain name you want to manage. Click the gear icon in the top right, and click "Manage DNS".
From the DNS Management page, click the "Edit Record" button (pencil icon) to change your CNAME Record on the line that reads "www" in the "Name" column.
Change the "Points To" value to domain.multisiteadmin.com and click "Save."
(Copy and paste, or type domain.multisiteadmin.com - Do not use your actual domain name)
Scroll to the bottom of the DNS Management page, to the "Forwarding" section, and next to "Domain", click "Add"
Type the domain you are using, but with "www." in front of the domain. This is a necessary step to ensure the site always loads with the www prefix. For the "Forward Type", choose "Permanent (301)". For the "Settings" choose "Forward Only" and be sure to check the box to "Update Nameservers and DNS settings to support this change". Then click "Save"
On your Williamsprobuilder site, enter the domain name on your Settings tab and click the "Save All" button. It will then take up to 72 hours for the change to fully take place sometimes instanly
NOTE: You do not need to change your domain's nameservers, only the CNAME Record and Domain Forwarding. You also do not need to purchase any hosting services for the domain.