Turn off the water supply to the shower
Plug the drain to prevent small parts from falling in
Remove the faucet handle
Take off the escutcheon plate
Inspect the cartridge, valve stem, O-rings, and washers
Replace worn or damaged parts
Clean mineral buildup from the valve components
Reinstall the cartridge or valve stem
Reattach the escutcheon plate and handle
Turn the water back on
Test the faucet for leaks
Replace the entire faucet assembly if repairs do not stop the leak