Before reading this blog, ensure that you have a Google Analytics account set up. If you are struggling with this, check out this link.
Once you have your Google Analytics Tracking ID, go to the 'Home' menu of you Squarespace site. Then click 'Settings' > 'Advanced' > 'External Services'. This will take you to a page looking like this:
In the box with 'xxxxxxxxx-20', enter your tracking ID and then click 'Save'. It is not uncommon for it take up to 24 hours for your website's statistics to show up on your Google Analytics dashboard as it takes time for the search engine to crawl your site and pick up the Analytics Account Number.