1. Log into cPanel (http://www.YourDomain.com/cpanel)
2. Go to Files menu
3. Click on FTP Account.
4. You will see Add FTP Account.
a) Login is the unique username that you want for that account.
b) Select your main domain for Domain.
c) Enter Password.
c) Directory is your sub domain directory. (Example: /home/abc/public_html/support/ )
5. Click Create FTP Account.
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 just want the new user to be able to access a sub-domain, just set the directory box to the same value as the sub-domain's root name, which is listed in parenthesis on the sub-domain section of this control panel.
Please be aware that the new user will have read/write access to the directory you choose and every directory below it.
For example, if you choose to add the user john, and give the user access to /home/abc/public_html/john directory, the user will be able to ADD, EDIT, RENAME and REMOVE any file or directory in /home/abc/public_html/john.