[Plugin: WP Page Numbers] not understanding wp-page-number instructions

I added <?php if(function_exists(‘wp_page_numbers’)) : wp_page_numbers(); endif; ?> to function.php. But page number is not shown…

I don’t understand where exactly the following steps should be done, Do I need to do this if I am happy with the available looks of wp-page-number?

Theme folder name

1. Copy one of the theme folders
2. Edit the ‘wp-page-numbers.css’ to make your changes
3. Rename your folder copy to a theme name
4. Upload your folder into ‘wp-content/plugins/wp-page-numbers/’
5. Set your ‘Theme folder name’ in admin ‘Settings / WP Page Numbers’
6. ‘Update settings’. Done!

Thanks!

https://www.ads-software.com/extend/plugins/wp-page-numbers/