1. # sudo apt-get install apache2 openssl
2. # a2enmod ssl
3. # a2enmod rewrite
4. # mkdir /etc/apache2/certificate
5. # cd /etc/apache2/certificate
6. # openssl req -new -newkey rsa:4096 -x509 -sha256 -days 365 -nodes -out apache-certificate.crt -keyout apache.key
7. /etc/apache2/sites-enabled/000-default.conf
8. service apache2 restart

1. # vi /etc/apache2/apache2.conf
2. add the followings at the end of the file:

      <Directory /var/www/html>
	AllowOverride All
      </Directory>
  

Let's Encrypt issues certificates through an automated API based on the ACME protocol. An ACME client is required and certbot client is recommended for such a program. It is very simple to install. Reference: https://letsencrypt.org/getting-started/

  $ sudo snap install --classic certbot
  $ sudo ln -s /snap/bin/certbot /usr/bin/
  $ sudo certbot --apache   # select 000-default.conf with https
    # certbot modify 000-defaults.conf accordingly. 
  $ sudo certbot renew --dry-run

add a new domain in the same server. (multiple domains/certificates in a server)

  # cd /etc/apache2/certificate/
  # openssl req -new -newkey rsa:4096 -x509 -sha256 -days 365 -nodes -out apache-certificate2.crt -keyout apache2.key
  # cd /etc/apache2/sites-available/
  # emacs 000-default.conf
  # certbot --apache --duplicate # it modified 000-default.conf
  # certbot renew --dry-run

1. Download and unzip doc.uk.to.zip
   1. /etc/ssl/ca_bundle.crt
   2. /etc/ssl/certificate.crt
   3. /etc/ssl/private.key
2. /etc/apache2/sites-enabled/000-default.conf
3. service apache2 restart