Kiwiz compatibility

Hi guys, I’m trying to have this plugin work with Kiwiz woocommerce invoice payment certification plugin in France to have sequential order numbers.
I read this:

If you like to make your other plugin ( invoice / payment ) compatible with Sequential Order Numbers for WooCommerce, please make below tweak. Instead of referencing $order->id or $order->get_id() when fetching order data, use $order->get_order_number()

Any idea where to insert this code?

Thanks heaps.

Gwen