The madness in this excellent short story is conveyed through the peculiar obsession that the narrator has with the yellow wallpaper that covers her bedroom walls and which she comes to relate with more and more. As she stares again and again at this pattern, she comes to see a woman, or many women, behind the wallpaper, seemingly pushing against it and trying to get out. Note how she describes this sight:

The front pattern does move--and now wonder! The woman behind shakes it!

Sometimes I think there are a great many women behind, and sometimes only one, and she crawls around fast, and her crawling shakes it all over.

The madness is conveyed through the narrator's increasing identification with this woman, until, at the very end of the story she identifies so strongly with her that she becomes that woman, walking around and around the edge of her room, physically expressing the mental and psychological entrapment she has experienced.