Download from ZeroSSL
Download openssl library from https://wiki.openssl.org/index.php/Binaries
set path to the extracted directory
execute following command to convert the crt file to pfx file so that it can be recognized by IIS certificate manager
openssl pkcs12 -export -out certificate.pfx -inkey private.key -in certificate.crt -certfile ca_bundle.crt
import the pfx in iis
--detailed steps coming soon...