Before enabling SSL, make sure your vhost has been activated ( see instructions here: https://gilbert.busana.lu/?p=165 ).
If let’s encrypt’s certbot is not installed, proceed here:

sudo apt install certbot python3-certbot-nginx

To install the let’s encrypt certificate, you need to have your external DNS server pointing to your reverse-proxy/web-server. To install the certificates and have certbot modify your vhost config file for you, do:

sudo certbot --nginx -d www.mydomain.com

If failed, try:

certbot install --cert-name www.domain.com
nginx -t
systemctl restart nginx

If everything went well, your certificate is saved at: /etc/letsencrypt/live/www.mydomain.com/fullchain.pem
The key is saved at: /etc/letsencrypt/live/www.mydomain.com/privkey.pem
Certbot also modifies your vhost vonfiguration file and adds SSL on port 443 and redirects unencrypted (http) traffic to secure sockets (https). Check your configuration file here: /etc/nginx/sites-enabled/www.mydomain.com