If you have have not done so already, download the Check-in Scan app:
The owner will have left somewhere in the house (normally behind the front door) a similar sheet to the following:
Now that you have arrived at the property, please register within 24 hours from arrival.
Open the Check-in Scan app on your mobile and click on "I'm a guest"
Position your mobile phone in front of the QR code, as if you were going to take a photo, and click on "Scan QR code".
In a moment, you will see a screen with a red cutout box, make sure that the entire code is within the red box, move the camera back a little if necessary.
Once you have gotten to this point, you only have the follow the instructions from the Check-in Scan app to scan your documentation and send it to the authorities.