From the Cpanel control panel, select the icon that says FTP accounts.
Enter the login name and password you would like for your FTP user.
Select the directory you wish to give the user user have access to. Web site files are held in the directory called public_html, so if you want to set up a user that only has access to the site files, use this directory name - the full path will be /home/yourusername/public_html
Enter the desired quota and click the Create FTP Account button.