The following are the steps to create a Subdomain FTP Account in cPanel
Log in to cPanel (http://www.YourDomain.com/cpanel)
Enter the "Files" menu, then click "FTP Account".
You will see "Add FTP Account"
a). "Login" is the same as your subdomain name. (Example: If your sub domain is support.exabytes.com, enter "support" as the login.)
b). Enter the password.
c).Directory is your sub domain directory. (Example: /home/exabytes/public_html/support/)
Click "create" to add this subdomain FTP user.
NOTE:
If you enter a single / in the directory box, the new FTP user will have access to the entire public_html directory and all directories under it. If you only want the new user to be able to access a specific subdomain, simply set the directory box to the same value as the root name of the subdomain, which is listed in parentheses in the subdomains section of this control panel.
Please note that the new user will have read/write access to the directory you selected and any directories below it. For example, if you choose to add ftp user john, and give the user access to the directory /home/exabytes/public_html/john, then the user will be able to add, edit, rename and delete files or directories in /home/exabytes/public_html/john.