Finding your theme selectors is not that hard. Most WordPress themes use the same theme selectors so there is no need to change them but if you need to change one or all theme selectors, this guide will help you.
First, make sure that you are viewing a single post on the site you are setting up Auto Load Next Post on. Then depending on your browser, you need to open developer tools. Or simply Right-click on the post title and select Inspect Element.
This will allow you to see the theme selector your post title is using, normally a main heading with a class applied.
<h1 class="entry-title">Development Report #1</h1>
When entering the theme selector in Auto Load Next Post settings, make sure you put the HTML element before the theme selector. In this case the HTML element is h1 followed by .entry-title for the class.
The same approuch is done on all theme selectors required. Once entered in the settings, press Save.