How do I create a 'Thank You' page URL for tracking conversions?

I’ve spent a month building an ecommerce site for a client, and now that I’m setting up tracking–which obviously would be easy to setup in the largest ecommerce platform–it turns out I can’t track the checkout page at all and there’s endless ambiguity about how to track conversions at all.

This is bizarre.

I set this site up specifically for Google Ads, so now what?

I’ll try /checkout/thank_you and cross my fingers.