Clean the fan unit by vacuuming it out or wiping it down with a rag.
Adjust the fan inside the wall so that the metal duct connects flush to the fan housing.
Select a cover that matches your particular model of exhaust fan. If you don't know the model you can purchase a generic spring-loaded grill cover from a hardware store. Measure the dimensions of the fan so you can match the cover to them.
Attach the hooks, located at the ends of the vent springs, to the inside duct of the fan. There should be one spring on either side of the grill cover.
Push the grill cover up to the top of the ceiling and the springs will lock into place, covering the fan.