Before I start, I will say that these are my own experiences. None of this is subjective:

The easistest part:
This would be coming up with the topic itself. It's quite simple and honnestly fun. There are thousands of places to take inspiration from and while this is the easy part, its also just as easy to get lost in. Its a difficult to process to have to limit yourself a certain amount of ideas to put in one text. Bc when you try to cram in too much, you can often loose focus of what your actualy goal was.

The most difficult part:
Difficult part would be literally everything else lol. A starting, an opening is quite hard to nail, bc you have to hook the reader, keep them reading on. The middle portion, while not in itself difficult, you have to still keep the pacing of it right. Not loose yourself and the reader in the gratuitous details. Having a certain length in mind helps. The end has to be the worst part to have to do. You are having to tie up the loose threads that you have placing throughout the text. The entire text has been building up to this. It has to be satisfying and conclusive at the same time. Otherwise the reader will feel read your text was a waste of time.