To connect Calm with Google Fit, you will need to download the Calm app on an Android phone or tablet. Once in the Calm app, tap the 'More' button in the bottom right corner of the app. In the side menu that opens, tap Profile, then Settings, and you will see 'Save to Google Fit'. Toggle that option on (so the slider is blue instead of grey). You will see a prompt to confirm that you'd like to save sessions to Google Fit.
Please see this support article for more details about using Google Fit: https://support.google.com/fit/?hl=en