Once your manager publishes the open shifts, you can easily pick them on your iPhone app. On your iPhone app, tap on ShiftPlanning at the bottom and on the next page, tap on Pick up.
Navigate to the date on which the open shift was posted by using the small arrows. Once you view the shift, tap on the shift and you can view all the shift details and request to work on this shift.
You can always cancel your request anytime until it is not approved.
If you have any further questions, don't hesitate to contact at firstname.lastname@example.org.