In Sudzy Settings -> Preferences, turn on "12. Allow customers to sign and leave tip."
You may want to get the iPad kiosk swivel to make it easier to turn the iPad to the customers. You can buy the swivel here.
In the Dropoff state, when you click Pay and either "Swipe Card" or a saved card, the tip page will appear.
Turn the iPad around to face the customers. Customers can add either No Tip, $2, $3, or $5, or Custom. Customers will then sign and press "Done Signing".
The tip will be charged when you charge for the payment.