If it has not already been created, you need to create a Google Tag Manager account and a container.
After that, simply copy the scripts provided when creating the Google Tag Manager container into the theme's skeleton.html file (from Backoffice > Ecommerce > Theme > Edit HTML/CSS).
Insert the code snippet
<script> into the
<head> element, preferably next to the opening
<head> tag, but after any statements in the dataLayer.
Then insert the code snippet
<noscript> immediately after the