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 menu icon in the top left corner of the app home screen. In the side menu that opens, tap 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