Look into the add_shortcode function, which can be part of the plugin (although I don’t recommend modifying other’s plugins, since your changes will be overwritten if they send out an update.
So you would add this function to your ‘child theme’ function.php
function add_my_shortcode()
{
add_shortcode('your-shortcode-name', 'the_function_for_your_shortcode');
}
add_action('init', 'add_my_shortcode');
..and here is the function that takes the [your-shortcode-name] and outputs some text:
function the_function_for_your_shortcode()
{
return 'here is my shortcode text'; // Shortcode output should be returned
}
Now, whenever you put the [your-shortcode-name] in a post/page, it will be replaced with the ‘here is my shortcode text’.
A simple (and somewhat useless) example, but it should get you started.