1. Define testing goals and intentions first :
You need to determine two things:
Why the client is sponsoring the study.
What they intend on doing with the results.
2. Generate very specific questions the study must answer
The easiest way to come up with tasks to test is to start with the questions the study should be answering.
I used to focus on asking clients to tell me their “mission critical tasks”, but that can limit your ability to come up with tasks that actually touch and test attributes that clients believe are problematic.