This article covers generating a wildcard CSR that will cover a domain and all of its subdomains.
1) Log into cPanel or WHM and navigate to the following location.
Home > Security > SSL/TLS > Generate, view, or delete SSL certificate signing requests
Home > SSL/TLS > Generate an SSL Certificate and Signing Request
2) In the "Domains" section, add the following lines, replacing example.com with your domain.
3) Fill out all other required fields appropriately and click the Generate/Create button.
Once done, the resulting CSR can be provided to a Certificate Authority. After they have issued the SSL, it can be installed in the following locations of cPanel and WHM.
Home > Security > SSL/TLS > Manage SSL sites
Home > SSL/TLS > Install an SSL Certificate on a Domain