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
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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.
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After cloning this template, add your own steps in order to populate the reservation with any data required for your use case.