There are some elements that you may wish to consider when writing imaginative responses in response to Module C questions. First, consider the stimulus and the question, and then choose a general theme (e.g. power) that you would like to focus on in your imaginative piece. This is especially possible when the stimulus is more general and does not express a particular theme, in which case you would instead have to develop your imaginative piece according to the specific theme in question. In choosing a theme, you can use your study of Module C (and perhaps the other English Advanced modules) as a reference point. The theme should also be one that you are confident with and are able to comfortably use as the foundation for your imaginative response.
Subsequently, you can explore the ways in which the authors of your prescribed texts have approached their own writing. This includes their use of particular techniques as well as their writing style. With this understanding, you can find inspiration from your prescribed texts and embed some aspects of those texts into your own writing according to the focus of your own imaginative piece as well as the needs of the question. From this point, preparing simply becomes a matter of engaging in substantial practice, regularly seeking feedback from your teacher regarding your writing and implementing any suggestions that they might offer, allowing you to maximise the quality of your writing.
I hope this helps!