1. add repository to system.

sudo add-apt-repository ppa:certbot/certbot

1. updating apt.

sudo apt update

1. installing certbot

sudo apt-get install python-certbot-apache
// or
sudo apt-get install python-certbot-nginx

1. installing SSL by certbot

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

1. (Optional) we need to set auto renew the sertificates by crontab -e

sudo crontab -e
// and add this code deep below: (this will be executed everyday at 1 a.m.)
1 1 * * * certbot renew --post-hook "service nginx restart"

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References :

• https://www.domainesia.com/panduan/cara-install-lets-encrypt-di-vps/#Nginx

• https://www.onepagezen.com/letsencrypt-auto-renew-certbot-apache/