$ sudo htpasswd -c /var/www/your_domain/.htpasswd an_user
$ sudo htpasswd /var/www/your_domain/.htpasswd another_user

$ sudo vim /etc/apache2/sites-available/your_domain.conf
<VirtualHost *:443>
  ServerAdmin webadmin@localhost
  ServerName your_domain
  DocumentRoot /var/www/your_domain
  ErrorLog ${APACHE_LOG_DIR}/error.log
  CustomLog ${APACHE_LOG_DIR}/access.log combined
  <Directory "/var/www/your_domain/password_protected_folder">
    #Options Indexes Multiviews
    #IndexOptions FancyIndexing SuppressColumnSorting SuppressDescription SuppressLastModified SuppressHTMLPreamble HTMLTable IconHeight=20 IconWidth=20
    AuthType Basic
    AuthName "Restricted Content"
    AuthUserFile /var/www/your_domain/.htpasswd
    Require valid-user
  </Directory>
</VirtualHost>

$ sudo apache2ctl configtest
Example Output
Syntax OK

$ sudo a2ensite your_domain.conf

$ sudo systemctl reload|restart apache2.service