If the default edX theme is being used, get the GA key from the customer and enable it in server-vars.yml.

EDXAPP_GOOGLE_ANALYTICS_ACCOUNT: "UA-DUMMY"

If using a custom theme, place the script in themes/templates/theme-google-analytics.html

https://github.com/Stanford-Online/edx-theme/blob/master/templates/theme-google-analytics.html