In order to add a Amplify to your Shopify site, you must first login to your Shopify account.
Once logged in, navigate in the left hand menu to "Online Store".
In the fold-out menu, select "Pages.
Choose the page where you'd like to add your Amplify. In the content editor, switch to Show HTML (the icon with the red arrow below). Then simply paste the code snippet from Notified and save your changes.