I would begin with an introductory paragraph explaining three key indicators of the narrator’s insanity: the change in style of the narrative, the change in the narrator’s attitude and the transformation of the wallpaper itself.
In the second paragraph you can comment on how the narrative changes from a detailed account of the house itself, the gardens and life beyond the room to a fragmented account of the narrator's internal hallucinations. From –
The most beautiful place! It is quite alone…
…outside you have to creep on the ground, and everything is green instead of yellow.
The third paragraph could illustrate how the narrator goes from begrudgingly obeying her husband to defying him by the end. As she concludes-
I had to creep over him every time!
Paragraph four could show the wallpaper changing from the narrator’s focus, to fascination, to prison-
I've pulled off most of the paper, so you can't put me back!
The fifth paragraph concludes the essay explaining that these three elements demonstrate the narrator’s decline into insanity.