How do I enable SSL on Apache Ubuntu and set up two-way authentication?

On Ubuntu, install Apache and OpenSSL, enable the SSL module with `a2enmod ssl`, and edit `/etc/apache2/sites-enabled/default-ssl.conf` to specify `SSLCertificateFile` and `SSLCertificateKeyFile`. For mutual authentication, add `SSLCACertificateFile` pointing to the client certificate, `SSLVerifyClient require`, and `SSLVerifyDepth 10`. Enable the default SSL virtual host with `a2ensite default-ssl` and restart Apache.