you can visit the Stadia website and sign in with your Google account (https://stadia.google.com/)
Or just download the Stadia app on iOS or Android.
You can play on your TV using a Chromecast Ultra or on some Android Phones or mostly everywhere the latest Google Chrome browser runs.
Please keep in mind that, although the Stadia Free Trail is free of charges, it could be possible that you have to enter a valid credit card that will be. charged with a dollar to verfiy it.