Newbie Question

Hi,
Ok I’m new to this and hope you can explain in simple terms how to get this to work with my theme and which files I need to edit, I understand this is low level stuff an apologise for numbty questions..

Your plugin installs, I can see the subtitle in my admin (standard) post but its not displaying on the page preview or output. What do I need to alter?

the functions.php? and add which code whereabouts?
Do I need to add something to my child theme?

Sorry if this is a very basic question ??