Note: If you are running the Jupiter cPanel theme then please refer to the following guide as the steps are slightly different:
How to create a subdomain in the cPanel Jupiter theme
How to create a subdomain in cPanel
This demo assumes you've already logged in to cPanel, and are starting on the home screen.
Now let's learn how to create a subdomain.
Click the Subdomains icon.
Enter the prefix of your new subdomain.
A directory name the same as your subdomain prefix will automatically appear... although you can change this if you want.
Click the Create button.
That's it! A new subdomain has been created.
This is the end of the tutorial. You now know how to create subdomains in cPanel.
Make sure the domain name your subdomain will be associated with is selected.
A new folder called clients has also been created, which is where you would upload files for this subdomain.
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Please note, the above tutorial is for cPanel's Paper Lantern theme.
cPanel have recently introduced changes to their themes, if you are using the Jupiter them, sub-domains are now accessed via Domains > Create. You can then enter the sub-domain you wished to create as per the guide here: