Log in to your Web Admin Panel and then click on the "Domains" tab and click on "Web". Find the subdomain and then click on the "Edit" link.
Make sure "Fully Hosted" is selected, then choose the name of the user for all the user fields, and that the Web directory is "/home/username/subdomain"
Click "Save Web Service" and just wait for the changes to be completed.
When the changes are completed, the user should have a directory "subdomain" when he logs into FTP, and files he uploads there will be made available on the web. This simple setup will work if you do not wish to "share" files between your user account and the new user account; if you want to access the files for that subdomain you would have to login as that user.
Perl / MySQL / HTML+CSS