How do I comment on the ‘stylistic features’ of a set text?

Quite simply, you comment on anything that’s interesting. A wise man once said to me that best way to pick out what’s interesting about a text would be to work out what you would remove to render the text into its most basic or boring form. Anything you remove is worth commenting on.
The second, and perhaps harder part, of answering a style question is working out what interesting bits of the text are pertinent to the content of the text or what the question is asking for. Here my advice is look at each feature and work out what it does to the passage. For example (hypothetically), a passage with lots of verbs has a lot of action and might intensify the pace of the passage, or something within the passage, like a battle or someone running. In the answer mention how the action in the text is emphasised by the run of verbs. To refine the answer, I would cluster similar points together, such as the polysyndeton (lots of connecting words) between all the verbs in the passage further increasing the intensity of the action.