But I would recommend the vsftpd method mentioned previously.
But, most importantly, there is no need to do this since, by enabling SSHD on a Ubuntu system, you get access to the FTP subsystem in SSH. Also, it's way more secure. So, the question is why would you want to enable any other FTP method besides SSHD??
You can see an excellent guide below
http://ubuntuforums.org/showpost.php?p=3138955&postcount=1
Here is the method using ProFTPD:
http://ubuntuforums.org/showpost.php?p=429783&postcount=1
But I would recommend the vsftpd method mentioned previously.
But, most importantly, there is no need to do this since, by enabling SSHD on a Ubuntu system, you get access to the FTP subsystem in SSH. Also, it's way more secure. So, the question is why would you want to enable any other FTP method besides SSHD??