How to implement in NewsPaper Theme

Hi, I am using NewsPaper WordPress theme

Hello, I used Multiple Most thumbnail Plugin to add multiple thumbnail for single post. In Module 6 I found this code which display the first thumbnail
<?php echo $this->get_image('td_100x70');?>
In theme documentation the way to show secondary thumbnail the code is

<?php if (class_exists('MultiPostThumbnails')) :
    MultiPostThumbnails::the_post_thumbnail(
        get_post_type(),
        'secondary-image'
    );
endif; ?>

Please tell me how can I use this plugin in NewsPaper theme.

https://wordpress.org/plugins/multiple-post-thumbnails/