Clear and well-written. Some suggestions are given for minor edits.
Setup and Play: "the GM tells the player to draw one or more insanity cards [based on the table below.]"
Tells: "[For example,] once a player has the Paranoia card..."
Descents: "Cards played as Descents need not match the Tell they are played underneath." I would bold or otherwise highlight this, as it is important and easily overlooked.
"so the number in the bottom right corner": I assume they're all 1. If so, I'd say "the number one..."
It would be useful to see the numbered effects close up in an example. With the Delusions shown, you could highlight the 3. box so we see how this ties together, and/or you could show a close-up of the first two Paranoid boxes in the Howard example.