QGroundControl will guide you to place the vehicle on a flat surface and keep it still.
The calibration steps are:
Click the Gyroscope sensor button
Place the vehicle on a surface and leave it still.
Click Ok to start the calibration.
The bar at the top shows the progress:
When finished, QGroundControl will display a progress bar Calibration complete
If you move the vehicle QGroundControl will automatically restart the gyroscope calibration.