When you arrive on site, once you have met your on site manager or team leader, open the app and tap the "clock in" button on the shift screen - you'll then see your unique QR Code.
Ask the manager/team leader to scan you in to generate a clock in record for your timesheet. With clocking out, again, meet you manager/team leader. Inform him/her you have finished your shift ask him to scan you out with the same method above.
If the in-app scanner isn't working or the employer doesn't have their phones with them, you will be able to clock in manually at the bottom of your QR Code screen. However, manual clock in records are not guaranteed to be paid weekly because the specific records will need to be confirmed with employers.