Hi,
Follow the rule of thumb: Input, Process, Output.
If you have already collected the requirements from the client, then you would be having all the details, just group the information into any of the above said category.
Input : Input to the system.
Process: The process involved - the business logic.
Output : The output expected from the client.
Before this
you should have identified smaller components of the system.
Collect all the process involved, the boundary conditions, validations, business rules regarding the functionality.
If you have the domain knowledge, you can prepare a questionnaire to be answered by the client.
I can provide more information on seeing your reply.
Hope this helps
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-Vijay
The Hand that gives, Gathers