In answering a question like this it is important to pay very close attention to the examples of Winston's writing that are in this novel so that stylistic features can be identified and replicated. There is a perfect example of Winston's writing style at the beginning of Chapter 6, when he narrates his account of visiting a prostitute. Note how he makes use of very short sentences to gradually tell the story:
It was three years ago. It was a dark evening, in a narrow side-street near one of the big railway stations. She was standing near a doorway in the wall, under a street lamp that hardly gave any light. She had a young face, painted very thick.
The sentence structure is very simple as Winston builds up his account, using lots of repetition of similar structures to develop his picture, such as the use of "She..." to begin sentences. It would be a good idea to write a journal entry from Winston's perspective abotu Katharine, his former wife. This chapter gives lots of insight into his relationship with her, and you could write about their physical relations. This could be an example:
Every time I touched her she froze like a plank. Once, during the night, when I was on top of her, I saw her expression of pure horror and loathing. It was enough for me to be near her for her to become uncomfortable. It seemed such an effort for her to be close to me. It seemed to cause her physical pain to allow herself to be touched by me.
This could be one idea of what you might write and how you might write it, but there are plenty of other areas that would be great subjects for a supplementary note from Winston. What must be evident is the kind of horror and pity that Winston captures as he writes about the past and his memories in order for the reader to understand his position better.