If you have already set everything up you only need to do this:
sudo htpasswd -d /etc/vsftpd/ftpd.passwd ftpuser
Then you can create their directories:
mkdir /var/www/ftpuser
chmod -w /var/www/ftpuser
mkdir www/ftpuser/www
chmod -R 755 /var/www/ftpuser/www
chown -R vsftpd:nogroup /var/www/ftpuser
And restart the service.
sudo service vsftpd restart