Adding some text to a specific email with WooCommerce Pretty Emails
If you need to add some text to one specific email like extra terms & conditions to the completed order email you will need to "hook" and action. In that function all you have to do is check that you're in right email then display your text.
The following snippet will help you display a paragraph in the completed order email.
You have to possible location to put that code in :
1 - In your theme / child theme in functions.php
2 - Create a new "mu plugin" which basically is a php file that you will store in your wp-content/mu-plugins/ folder (you may have to create the folder if it doesn't exists.