You can start measurement in Firstbeat Life application by selecting Start new measurement.
Then select the length of the measurement: you can choose between 1-5 days. The first measurement should be 3 days so that the analysis can establish your baseline for accurate results.
The application will guide you how many electrodes you need for the selected number of days.
Make sure that your device is paired with the application before starting the measurement. More help how to pair can be found from this article: Pairing Bodyguard 3 with bluetooth connection
You can't start a new measurement if the device's battery level is low. In this case you need to charge the device battery. See here more details on how to charge the battery: Recharging Bodyguard 3 device
Once the device is paired and charged, you can start your measurement.
If you need any further help with this, don't hesitate to contact Firstbeat Support.