Download the Qld Gov't Check In App (if you don't have it already) - click here
To scan one of the QR Codes as you enter a venue, just bring up your phone's camera, and hover over the QR Code. Your phone will recognise the QR Code, and open the Qld Gov't Checkin App. Complete the information and your are done (you can add family members/friends to your check-in).
Don't try to take a photo (like I did first time). All you get is a photo. Just hover until it recognises the code.