Turn off the water supply to the shower
Open the faucet to drain any remaining water
Remove the handle screw and take off the handle
Remove the trim plate or escutcheon
Remove the retaining clip or screws holding the cartridge or valve stem
Pull out the old cartridge or valve stem
Install the new cartridge or valve stem
Reinstall the retaining clip or screws
Reattach the trim plate or escutcheon
Reinstall the handle
Turn the water supply back on
Test the faucet for leaks and proper operation