Just follow the following steps:
# mv myprivatekey.key /etc/ssl/private
# mv mycertificate.crt /usr/local/share/ca-certificates/
# mv CAcert.crt /usr/local/share/ca-certificates/
# update-ca-certificates
# ll /etc/ssl/certs/ | grep -i mycert
# vim /etc/apache2/sites-enabled/default-ssl.conf
----------------------------------------
[...]
SSLCertificateFile /etc/ssl/certs/mycertificate.pem
SSLCertificateKeyFile /etc/ssl/private/myprivatekey.key
SSLCertificateChainFile /etc/ssl/certs/CAcert.pem
[...]
----------------------------------------
# systemctl restart apache2.service