These are general instructions. You can get instructions specific to your Domain Host.
aka Domain Manager, Name Server Management etc.
You might have to enable advanced settings to access this page.
Create a new CNAME record
In the Host Name/Alias field:
enter the subdomain from your URL
If you put 'signup.yoursite.com' in your LaunchRock Domain Settings, enter 'signup'
In the Value / Destination field:
enter host.launchrock.com
Save the new settings
Once you've modified your CNAME records, it may take up to 72 hours for changes to update live