Steps

1. sudo mkdir -p /var/www/rex.com/
2. sudo chown -R ubuntu:www-data /var/www/rex.com/
3. sudo chmod -R 755 /var/www/
4. cd /etc/apache2/sites-available/
   sudo cp 000-default.conf rex.com.conf
   sudo nano rex.com.conf
5. sudo a2ensite rex.com.conf
6. sudo service apache2 restart

Reference:

1. http://diary.taskinghouse.com/posts/664386
2. https://nouveauframework.org/blog/vhosts-running-multiple-sites-on-a-single-aws-ec2-instance/