A PDS specifies a problem, not a solution. A PDS does not pre-empt the design process by predicting its outcome. Rather, it defines the task by listing all the conditions the product will have to meet. This can involve a good deal of research, into market conditions, competing products, and the relevant literature including patents.
When you write a PDS, you are defining something that does not yet exist. But for practice at thinking this way, it can help to look at an existing product and work out what its PDS was.