Essentially, the primary method of preparing for tasks that require a response(s) to unseen stimuli is to attempt a variety of questions that require you to do the same. Such questions can be from past exam papers as well as your own research and imagination, that is, if access to exam-style questions that are similar in nature is limited, you may be able to research particular texts that would be appropriate to use as stimuli for practice and/or use texts or parts of texts that you are already familiar with as stimuli. The more you practice, the wider the range of ideas that you can have and think of on the spot, and the easier it is to form relevant patterns.
An example of a stimulus would be an image showing or a text describing an abandoned and messy house. You could use such a stimulus to develop a creative piece about a person (whose age group you are free to determine) who suffered significant mental pressure growing up. The distinct or unique setting could be the person's mind, where their thoughts and feelings, and the manner in which those affect their actions, decision making and their life as a whole, can be explored. A particular aspect of the person's life (for instance, their inability to escape a certain unwanted situation due to contradictory circumstances or limitations - potential link to postmodernism in this case) could be highlighted as part of the overall pattern/paradigm. Of course, this is based on my understanding of your post, and you can definitely adjust your preparations according to the specific requirements of your task.
Even the fact that your school's English faculty staff like coffee could contribute to forming a stimulus that you can use in preparing for your task. One should avoid underestimating elements without properly looking into them first, regardless of how insignificant they may seem.
I hope this helps!