If you wish to scan tickets from your iPhone with iOS 11 or later or with any android smartphone you can click on More Options for the event and click on Event Detail Options from the top and the sidebar menu will slid right on the screen like the screenshot below. Click on Ticketing and Scan Tickets under that.
The browser will ask you for permission to use the Camera and if the browser supports camera you will see a screen like below
Once the camera is opened all you need to do is to point camera at the QR codes and if it's a valid ticket and not been used to check in Green bar will appear and if you scan same ticket again it will show Red bar with Already Checked In error message like the screenshot below.
If you do not have supported phone you can always manually check them in or out from the same page.
See the Attendee List and click on Check In or Check Out button to mark them as used tickets.
If you have any questions please email us on firstname.lastname@example.org