<?php
// You'd add the following code to the theme functions file (functions.php).

// Make sure that the function is called after the global $wcff is set; so we
// use the priority 12.
add_action( 'plugins_loaded', 'my_wcff_overrides', 12 );
function my_wcff_overrides() {
    // Make sure the wcff() function exists.
    if ( ! function_exists( 'wcff' ) ) {
        return;
    }

    // Load the custom builder class (file).
    //require_once get_template_directory() . '/wc-fields-factory/wcff-builder-custom.php'; // If NOT using a child theme.
    require_once get_stylesheet_directory() . '/wc-fields-factory/wcff-builder-custom.php'; // If IS using a child theme.

    // Modify Wcff::$builder so that it uses the custom class. Change the name
    // if it's not "Wcff_Builder_Custom".
    wcff()->builder = new Wcff_Builder_Custom();
}