Related to Certificates

Generate a CSR
https://www.digitalocean.com/community/questions/how-do-i-generate-a-csr-key

Create the self-signed certificate
https://help.ubuntu.com/lts/serverguide/certificates-and-security.html

Create thrid certificate using CSR
https://ssl.comodo.com/free-ssl-certificate.php
https://support.comodo.com/index.php?/comodo/Knowledgebase/Article/View/637/66/

Aditional information
https://www.ssl2buy.com/ssl-certificate-google-rankings
https://www.elegantthemes.com/blog/tips-tricks/how-to-get-a-free-ssl-certificate-and-why-google-is-forcing-you-to