Source: https://miketabor.com/how-to-install-a-lets-encrypt-ssl-cert-on-microsoft-iis/
1. Allow port mapping for http and https
2. enter hostname in IIS binding for http
INSTALL LET’S ENCRYPT ON MICROSOFT IIS WEBSERVER
Download the latest Windows ACME Simple (WACS) ZIP file from the Github releases page, https://github.com/PKISharp/win-acme/releases. At the time of writing this post, it is win-acme.v2.0.3.206.zip
Next extract the zip file to a folder of your choice. In this example I’m placing it in my C:\inetpub\letsencrypt folder.
3. Right click on wacs.exe and select Run as Administrator to start the Windows ACME Simple wizard.
4. Select N to create a new certificate.
5. Next you’ll be prompted for what kind of cert to create. Select 1 for a single binding of an IIS site.
6. Now select which site binding to configure. Since we only have one site on this IIS server there is only one option. Select 1.
7. Enter an email address to be used for notifications about any potential issues.
8. Finally, agree to the Terms of service and the Windows ACME Simple program takes care of the rest.
That’s it for installing a Let’s Encrypt SSL cert on IIS.