Step 1: Create the SSL Certificate

TLS/SSL works by using a combination of a public certificate and a private key. The SSL key is kept secret on the server. It is used to encrypt content sent to clients. The SSL certificate is publicly shared with anyone requesting the content. It can be used to decrypt the content signed by the associated SSL key.

We can create a self-signed key and certificate pair with OpenSSL in a single command:

$ sudo openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout /etc/ssl/private/apache-selfsigned.key -out /etc/ssl/certs/apache-selfsigned.crt

Modify the Default Apache SSL Virtual Host File

Next, let’s modify /etc/apache2/sites-available/default-ssl.conf, the default Apache SSL Virtual Host file. If you are using a different server block file, substitute it’s name in the commands below.

Before we go any further, let’s back up the original SSL Virtual Host file:

<IfModule mod_ssl.c>
        <VirtualHost _default_:443>
                ServerAdmin your_email@example.com
                ServerName server_domain_or_IP
                DocumentRoot /var/www/html
                ErrorLog ${APACHE_LOG_DIR}/error.log
                CustomLog ${APACHE_LOG_DIR}/access.log combined
                SSLEngine on
                SSLCertificateFile      /etc/ssl/certs/apache-selfsigned.crt
                SSLCertificateKeyFile /etc/ssl/private/apache-selfsigned.key
                <FilesMatch "\.(cgi|shtml|phtml|php)$">
                                SSLOptions +StdEnvVars
                </FilesMatch>
                <Directory /usr/lib/cgi-bin>
                                SSLOptions +StdEnvVars
                </Directory>
        </VirtualHost>
</IfModule>

Step 3: Enable the Changes in Apache

Now that we’ve made our changes and adjusted our firewall, we can enable the SSL and headers modules in Apache, enable our SSL-ready Virtual Host, and restart Apache.

We can enable mod_ssl, the Apache SSL module, and mod_headers, needed by some of the settings in our SSL snippet, with the a2enmod command:

$ sudo a2enmod ssl

$sudo a2enmod headers

Next, we can enable our SSL Virtual Host with the a2ensite command:

$ sudo a2ensite default-ssl

$ sudo service apache2 restart

Source: How To Create a Self-Signed SSL Certificate for Apache in Ubuntu 16.04 | DigitalOcean