The specific CSR generation procedure is determined by your choice of web server software. Generation instructions are available for the various web servers here
If the domain name you are purchasing the certificate for is hosted on our cPanel hosting platform, the relevant instructions are for cPanel 11. Please log into your cPanel and ensure that the 'SSL/TLS Manager' tool is present (under the 'Security' area of your tools) - if not, please advise so we can have this enabled for your account.
Once you have generated the Certificate Signing Request (CSR), to add this to your SSL order, follow the steps listed here