Remove all hay from cage. Place hay rack right next to litter tray. Place small piece of newspaper on litter tray, covered with old hay so the bun won't eat it. Place new hay on the hay rack next to litter tray so bun hops onto the tray to eat from the hay rack. If bun poops outside of litter tray, place the poop in the tray, and scrub any **** with white wine vinegar.
Changing tray. Clean it but keep a little of the newspaper. Place inside new newspaper, and add a clean batch of old hay. Place new hay in hay rack.
This method works if your entire cage isn't covered with hay to confuse the bun.