So it's pretty simple then:
It's a sub title plug in, so most people want it under the title.
For post titles, most themes it's single.php and you'd place it after:
<h1 class="entry-title"><?php the_title(); ?></h1>
Themes vary. Basically you are looking for the file that contains your post loop.
If you want it under your page titles, then find the page that contains your page loop. In many themes it's page.php. "
But it could be anything. Where ever your page loop is.
Seems like you are over complicating it in your avoidance not to give some kind of answer.
We all understand that not every theme names their files the same, but for people who are trying to learn and DIY, sometimes a step in the right direction is enough for them to go "Oh yeah. I know what file that's in.". Some kind of answer is better than "I can't tell you where to put it, it depends on where you want it to show"
A simple "under
<h1 class="entry-title"><?php the_title(); ?></h1>" would even be helpful. It would have saved me the hour of trying to make it work in the functions.php file.
By the way, I did all of that and it still didn't display.