To set up service charge (tips) for a booking you will have to go to the back office > Dashboard > Settings > General Price settings > Enable Service charge/Tips (towards the bottom of the page). Set this to "Yes" and you will see the following option "Show input box for Flat Tip: Yes/No". Select "Yes" to show an input box for the customer to enter the desired Tip amount on their own. 
You will also see the "Add new" button. Clicking on the button will open 2 options form: Amount input box for the amount of pre-defined tip and Value/Percentage choice for selection of the type of the service charge/tip. A common practice is to set 15% tip which will result in showing a quick selection button for a 15% service charge at the front end booking form third (final) step of the booking process. 
Create as many pre-defined service charge buttons as you wish.