If you are planning to set up this subdomain to point at our cPanel hosting [Click Here] for more detailed instructions
To make a subdomain, you simply need to edit your DNS records -- on our nameservers (if you were not on our nameservers already; then below DNS changes should be made with current nameserver provider), try these steps:
1) Log into your user account.
2) Go to "Domains" -> "My Domains"
3) Click on the domain name you wish to modify. This should take you to a new page.
4) Go to the "DNS Settings" tab
5) Select the "Use OnlyDomains Name Servers" option (If it isn't already selected) and click "Edit Zone Records"
From there, simply add DNS records where the "host" is your desired subdomain. (e.g. for "blog.mydomain.com", the host would just be "blog".)
The type and content of the record will depend on your hosting provider, please reach out to them if you are unsure.