Used to be called functional decomposition back in the old days.
So you have a set of business requirements and you need to work out what functions (user story, use cases, pseudo english, etc), workflow (BPMN, activity diagrams), rules (put them in the user story as acceptance criteria or in the use case as conditions), data (class diagram). I sometimes do a state-transition diagram to help with understanding of states in the workflow.
If you have time you can even mock up some screens and reports. I often use a state-transition diagram to show how the screens (states) and actions (transitions) to move between them.
Kimbo
I use the system of Critical To The Product, meaning if the product does not exhibit the feature or function, it will be useless. Important To The Product, meaning that the feature or function lends value to the product and the product would be less desirable or usable without it. Nice to Have In the Product, meaning that the feature or function is a good thing to have and will be extra furnishings on top of the important and critical parts. Those terms seem to resonate with the customers. If you drive the prioritization discussion on value, what features and functions provide the greatest value to the users and can that value be stated, it will help you along the way. I hope that helps you.
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