The difficulty of a paper is usually its innovation, that is, its bright spot. After all, the most valuable thing in a paper is innovation. So, once you find a good idea, you are usually halfway there. As for the "writing" of the paper, the most difficult thing is to express your innovation accurately and completely. You can generally easily explain what others have achieved, because these are what others have already said. For your own affairs, it is often not easy to express the core original ideas in your heart in words for others to listen to. You can read more excellent papers of others and see how they explain their core ideas. The above is just the most difficult part of the paper. In fact, the writing of the thesis is a complex system engineering, in which there are more than one difficulty. In short, write more, read more and think more. I believe I can write a high-quality paper one day.

The above is my original, I hope it will be useful to you!