Go to Booking →settings → extras
Click on New Extra
Give the extra a name
Set the price
Chose the display order
Set if the extra is quantity based
Enter how many minutes and/or hours will take to complete the extra
Link the services that will have this extra available
Upload an icon, the recommended size and format is 55pxx55px PNG with transparent background
Click create
Go to booking → extras
Choose an extra and click the edit icon
Edit what you must and click update
Go to Booking →settings → extras
Select an Extra and click on the delete icon
Go to booking → extras
Choose an extra and click the edit icon
In the section "Choose Services" Add or remove the service(s) you want this particular extra to have.
This is useful when you want only certain services to have a specific extra.