For Android, to enable the Tip feature,
Navigate to 'Setting' > 'Accounting', then select 'Enable' in the 'Tip' section.
It is always enabled for iOS and it's not possible to turn it off.
Once the Tip feature is enabled, when you go to the 'Checkout' screen and select an appointment to pay, 'Tip' field will appears in the payment item list. Then just enter the Tip value in that field, it will be reflected in the Tota field.
Also, the entered tip amount will be summarized and displayed in the accounting information on 'Accounting' > 'Payment List' and 'Profit Review'.