Don't spend time writing documents about tests; skip right to writing the tests themselves.
Write executable tests in a language that, ideally:
Is available on the platform where you need to run the tests
Has an implementation of the xUnit framework
Will interface with the test tool you're using (if any)
You're fluent with
Is the same language the system is being developed in
As you work out details with the customer and the rest of the team, put them directly into executable tests.
Work with the customer so she's comfortable reading and understanding the tests.
Use intermediate "bridge" modules to adapt the interfaces of system pieces to the interface desired for the test and to allow the test to execute through the user interface.
Refactor the tests and the interfaces to the bridge modules as you learn more details about the desired system behavior.
If you have trouble getting started, think about the actions, data, and expected results for various user scenarios.
Here's a chance for you to try out this format for yourself.
