Booker25 comes with flow templates that you can clone and adjust to your specific needs. This article explains the purpose of each flow template.
Default Reservation Flow Template
This flow is a very simple flow intended as a blueprint for flows that can make changes to a single reservation. This can be useful for example to pre-populate data on a new reservation. Some features that use this technique are:
Run a Flow when creating a new Reservation
The flow has a single variable named reservation which is available for input and output.
After cloning this template, add your own steps in order to populate the reservation with any data required for your use case.