How can I add the snippet for Google Ad Tracking to my site?

Once you have your Google Ad tracking snippet, you can add it to your site by dropping the entire snippet here:

Content > Message Editor > Custom Message on Order Confirmation Page.

Alternatively, if you want to use the snippet only for a specific payment method, you can add it to the WYSIWYG editor for that payment method here:
Settings > Payments > Edit >> Order Confirmation Message box