Using Let’s Encrypt Free SSL Certificates

apt install software-properties-common
add-apt-repository ppa:certbot/certbot
apt update
apt-get install python-certbot-apache
certbot-auto -d -d -d  -d

This should install the certificates for all the domains in their respective sections (443) in the apache config file.

Now the certificates should be renewed every 90 days: Test renewal by doing a dry run:
certbot renew --dry-run

If everything works, then configure a fully automatic renewal via a cron job, e.g.,
0 0 15 * * /usr/bin/certbot renew --quiet