Enable Apache SSL Xubuntu 18.04 Bionic LTS
The tutorial shows you step-by-step How to Enable Apache SSL in Xubuntu 18.04 Bionic LTS GNU/Linux – Step by step Tutorial.
And the Xubuntu 18.04 Apache SSL Enabling Procedure Described here is the Easiest one to Quick Start with Apache SSl on a Development Environment.
Finally, inside the Guide you find also instructions on How to Generate a Valid SSL Signed Certificate for FREE on Xubuntu.

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Open a Terminal Shell emulator window
Ctrl+Alt+t on desktop
(Press “Enter” to Execute Commands)In case first see: Terminal QuickStart Guide.
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Enabling Apache SSL.
sudo a2enmod ssl
sudo a2ensite default-ssl
sudo service apache2 reload
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Review the Apache Default SSL Configuration for localhost.
cat /etc/apache2/sites-available/default-ssl.conf
How to Generate a Free SSL Signed Certificate
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After you can Test Access the Domain over SSL.
For a Server (Example):
https://100.101.102.103
For a desktop:
https://localhost