Once your NS records are pointed to the Rackspace name servers, any modifications made through the Rackspace Cloud Control Panel will be authoritative for your domain. It is highly suggested to configure all DNS records in the control panel prior to switching NS records. This allows all records to be in place once the NS record has updated. Also be aware of the Time to Live (TTL) value for your previous Name Server. This time value, defined in seconds, will need to pass prior to DNS being live with Rackspace Cloud DNS.
If you are not still logged into the control panel, log in using the username and password specific to your account. On the top of the screen from the list of Rackspace Cloud Products select the DNS tab and select "Create Domain".
Select "DNS" from the product navigation tab at the top.
Click: "Create Domain".
To create your domain you will need to provide your domain name, an email address, and a TTL value for each record created. The TTL value dictates the length of time a DNS record is cached by any requesting server. For your purposes the 5 minute default value will work.
Insert the domain name you are adding, "example.com"
Supply your Email address, "user@example.com"
Set the TTL value for the domain, Suggested: 5 minutes
Next you will create an A record for the Load Balancer to begin routing live traffic.