Define the acceptance tests at a high level as soon as you read/see a story.
Write down the obvious things that come to mind.
If you get stuck, try this:
Write two tests for each action in the story, including descriptions of the inputs and system outputs:
- One that makes the action succeed
- One that makes it fail
When you've identified hidden assumptions, write them into the acceptance-test definitions.
Concentrate on what, not how, and don't sweat the details.
The following exercise gives you a chance to try this out for yourself.