How can we attempt a postmodern or reader response reading of a specific passage from a novel? I need guidance on what should be mentioned in a reader response commentary of a text.

Using reader response theory when approaching a text assumes that the reader is vital in creating meaning in a text. In this theory, there is a differentiation among the terms "text," which is simply the words on the page; "reader"; and "poem," which refers to the literary work produced by the text and the reader working together. Thus, both the understanding of the "facts" of the texts and the engaged mind of the reader are necessary to implement this theory in any sort of relevant capacity.
The reader plays his or her important role by bringing past experiences, knowledge, and thought patterns to the text. Therefore, when presented with any text, a reader should look for ways to connect that text to things that are already familiar and use those to form interpretations. All interpretations must, of course, be weighed against the text itself, but there are always places in literature that allow for multiple readings to take place. Reader response theory asks the reader to "hunt" for those places. So, when engaging with the text that you have, I would ask as you read: "What do I see that reminds me of my own experiences or knowledge base?" and "Where are there gaps in the text that allow for interpretation (such as character motivation, actions, etc.)?" Happy hunting!