<!-- BEGIN COPY / PASTE -->
    function my_custom_meta_box() {
        add_meta_box(
            'my_meta_box_id',          // Unique ID
            'Custom Field Title',      // Box title
            'my_meta_box_callback',    // Content callback, must be of type callable
            'post'                     // Post type
        );
    }
    add_action('add_meta_boxes', 'my_custom_meta_box');

    function my_meta_box_callback($post) {
        $value = get_post_meta($post->ID, '_my_meta_key', true);
        echo '<label for="my_field">Description for this field</label>';
        echo '<input type="text" id="my_field" name="my_field" value="' . esc_attr($value) . '" />';
    }

    function save_my_meta_box_data($post_id) {
        if (array_key_exists('my_field', $_POST)) {
            update_post_meta(
                $post_id,
                '_my_meta_key',
                sanitize_text_field($_POST['my_field'])
            );
        }
    }
    add_action('save_post', 'save_my_meta_box_data');
    <!-- END COPY / PASTE -->