Log in to your Google Tag Manager account and go to Admin > Install Google Tag Manager. Keep this page open in a tab so you can copy it’s contents later.
Go to Squarespace > Settings > Advanced > Code Injection
Go to your Google Tag Manager Tab and copy the Header script. Paste this code in the Header section in Squarespace.
Return again to Tag Manager and copy the Body script, Paste this code in the footer section in Squarespace and click on save.
You are now finished.