The option to link devices can be found in Settings > General > Connect Devices.
Tap the logo for the device you would like to link. Make sure you have downloaded the app for your chosen device as well as the Cronometer app.
Once you have chosen the appropriate device from the list, make sure you have chosen all the data you wish to import to Cronometer by using the toggles. Then tap the Link button.
You will then be directed to log into the account associated with your device. Once you have done this, press the back button to return to your Cronometer app.
Ensure you are not double counting your activity by navigating to the Profile section of the Settings tab and choosing the appropriate activity level. If your device tracks general activity as well as workouts, choose None. If your device only tracks workouts (e.g. Strava), choose Sedentary.
If you are unsure that your device linked, please navigate to the login page for your account with your device provider (e.g. your Fitbit account information page at https://www.fitbit.com/login#). Find your account information page to ensure that Cronometer is marked as one of your connected apps.
Please see the article on device troubleshooting for more information.
Google Fit/ Apple Health
If you have synced your account to Apple Health or Google Fit, set your activity level to None to ensure you are not double counting calories burned from your activities.