You captchalogue the other letter and combine it with your other note. Current items in inventory: Gun, Spamton glasses, Spamton Plush, Ghost image of Car, 2x Letter, Frog plush, and of course, trapdoor.
You then hop onto the little stool and look at the screen. At first there's nothing, but then, you start fiddling with the controls and suddenly, you see something: