Q:

How to setup Shopper Approved? I suppose to add their code to the check-out page to make their "survey" appear after the check-out process

Magento 1.9.1.0
We purchased the Shopper Approved package which basically asks the customers to leave a reference:

The first "quick survey" should appear on the CONFIRMATION page after a customer places an order and if they fill it out - it gets posted to Google, Facebook and who knows where else. If they fill out this initial survey - they will also get the
FULL survey by Email a few days later. 

What I need to figure out is - How do I add the code they gave me to the page that comes up for the customer after they placed their order?

Updated on February 7th, 2019: 
The path that worked for me in the end is: 

public_html/app/design/frontend/smartwave/porto/template/checkout/success.phtml

A:

Copy the PHTML-template 
template/checkout/success.phtml

from the Magento core theme folder 
app/design/frontend/base/default

to your own theme folder
app/design/frontend/YOURTHEME/default

