Do I need to do anything to turn this on?
No. It is always on, on both the iPhone and the Android app, for every screen.
From the guide Working offline.
Working offline
Both phone apps keep working when the signal does not. Fill in a form, log an expense, close a work order, take a meter reading, clock out — it is saved on the handset and sends itself the moment you are back on a network. You do not have to remember to do anything.
Read the whole guide: Working offline