Custom Action to display Release Notes only after an Upgrade/Patch Installation

[#1]

Can anyone please help me with a step by step guide to create a Custom Action on the SetupCompleteSuccess dialog to display Release Notes after the installation of a Patch or Upgrade?
The release notes will be a HTML file format I've been going around in circles a bit from web site to web site.
I don't know my way around Custom Actions so please assume I know nothing and please don't skip any steps.

I was following some of the steps on this URL but I don't understand some of them - plus I want to open a html file and not a text file:
https://community.flexerasoftware.com/showthread.php?196032-Open-a-text-file-before-the-end-of-installation&highlight=custom+action+readme

And the key bit about what program to launch isn't included.

Thanks in advance,
Ann Marie
(a QA Engineer who acquired the role of the maintaining our installer)

[Roman1]

Please look at the attached example.

[#3]

Hi Roman,

I am so sorry - I don't know how I missed your reply.
Thanks a million for your help. I will test out your sample project.

Kind regards,
Ann Marie