In order to track your website stats with Google Analytics, you need to obtain a tracking id for your website. Follow the steps documented by Google and copy your Tracking Id. It's in the form of UA-XXXXXXX-Y.
Go to Landly, choose the project you want to add Google Analytics and go to Settings > Integrations. In that page paste your tracking id to the Google Analytics input and it will be saved automatically.
Note: In order to see the integration on your website, you must publish your website.