Generate a self-signed SSL certificate

# apt-get install openssl
# openssl req -new -x509 -days 400 -nodes -out apache.pem -keyout apache.pem
# chmod 600 apache.pem
  

Load the certificate into apache2

Put the certificate in a suitable place:

# mv apache.pem /etc/ssl/certs/apache.pem
  

Make sure that apache2 is installed:

# apt-get install apache2
  

Enable the ssl module in apache2:

# a2enmod ssl
# a2ensite default-ssl
  

Tell Apache where the certificate is:
Look at "/etc/apache2/sites-enabled/default-ssl". Searching for the string "SSLCertificateFile" should lead you straight to the right section, where there are comments that tell you how to configure that variable. You probably just need to set the configuration to:

SSLCertificateFile     /etc/ssl/certs/apache.pem

Load the modified config file with:

# /etc/init.d/apache2 reload
# /etc/init.d/apache2 restart # probably unnecessary but healthy.