MY EYE FELL OUT!!!
Step 1: Materials
This costume doesn't take much effort, and I only needed to buy one thing...
The optic nerve is made of wool, the bandage is a proper eye-bandage (it's "out of date", donated to the boys by a first-aider friend), and the whole thing is held together with PVA glue.
Extra effects were applied with a touch of Kitewife's blusher.