Hi erni. After installing the WP-PageNavi plugin you would either create a plugin as Tom described in this post, or add this code to your child theme functions.php file:
add_action('generate_paging_navigation','generate_pagenavi_integration');
function generate_pagenavi_integration()
{
wp_pagenavi();
}
Then you would add this css to your child theme style.css file to hide the default theme pagination:
.page-numbers {
display: none;
}
Adding PHP Functions: https://generatepress.com/knowledgebase/adding-php-functions/
Adding CSS: https://generatepress.com/knowledgebase/adding-css/
Let me know if that helps.