[Plugin: Woocommerce] How do I implement tracking code into order confirmation?

Hi,

I’ve recently integrated my woocommerce website with an affiliate network. In order to track sales and award commission I have to implement a tracking code into the order confirmation page (thankyou.php) to be able to track validated sales and award commission. I’m new to code and this forum so don’t really know where to start, I’ve tried appearance > editor from within my wordpress dashboard but I can’t locate the thankyou.php file – only when I log into my ftp account I can. Should this code be implemented directly into here through ftp? & where about should it be implemented i.e. in the middle or at the end of the original code? Also best practise would be to create a child theme?
Also if anyone has experience with the plugin “code snippets” can it be done through this? I see that any code added is applied to the functions.php.

Any help on this would be greatly appreciated!