From the connections page, simply choose Google Analytics as your new data source.
If you're already signed into the Google account you want to associate Chartio with, you'll be prompted with a simple permissions request to allow access. If you're not, you'll need to sign in to your Google account before access is allowed.
Once you've granted permission, you'll be directed back into the Chartio interface. Google Analytics is now set up as a data source for your project, and you'll see it appear as a data source when you add a new chart.
Data sources are managed on a per-project level, so you'll need to uniquely connect any data source to every project you'd like to use it with.