Install the certbot:

apt install certbot python3-certbot-apache

Set the following in your vhost conf file:

ServerName mydomain.com
ServerAlias www.mydomain.com

Verify that your configuration is fine:

sudo apache2ctl configtest
sudo systemctl reload apache2

Make sure the dns entry in the external dns server points to your proxy server/web-server. Then launch the certification process:

sudo certbot --apache
sudo systemctl status certbot.timer
sudo certbot renew --dry-run

Note: nots ure about the last commands. Needs to be retested.