Remove the shower valve handle. Most handles have a cover in the center with a screw that you can remove with a Phillips screwdriver.
Remove the shower valve cover, if there is one over the valve, by removing the screws connecting the cover to the valve.
Inspect the valve cover piece for any damage, such as cracks in the metal, and discard the broken cover.
Remove the screws from the second metal plate connected to the valve, if there is one, and then remove the plate.
Place a new valve plate over the valve, and reinsert the screws into the plate.
Place a new shower valve cover over the valve, and reinsert the screws into the valve cover.
Reconnect the shower valve handle by reinserting the middle screw into the handle.