In your site's dashboard, click on "Settings", and in "Tracking Tools" click on "Manage".
Click on the "+ New Tool" button and select "Custom".
Paste your Plausible tracking code, choose which pages to add the code to ("All Pages"), select to "Load code once" (read more about this here), select "Head" as the place for the code and click on the "Apply" button.
Now you can go to your website and verify whether Plausible Analytics script has been added and to your Plausible Analytics account to see whether the stats are being tracked. See here how to verify the integration.