Log into your Direct Admin Panel.
Once you are logged in navigate to the Account Manager section on your panel and select "Subdomain Management"
This will open the Subdomain Manager, Once you are in the Subdomain Manager click on the green button saying "Add Subdomain"
Enter the subdomain you wish, and click the green button saying "Add Subdomain"
And you've set up your subdomain. If you encounter any issues with this process please don't hesitate to contact support.