Turn off the water at your home's main supply valve. This valve is typically located in the basement, but if your home doesn't have a basement, it will be on the exterior wall of your home.
Turn on the shower faucet to drain the water in the shower pipes. Turn off the faucet after you've emptied the pipes.
Place a rag over the drain to prevent any small parts from falling into it.
Pry the plastic cap off the handle, using a screwdriver, and then unscrew the screw inside the handle and pull the handle off. If there isn't a plastic cap, remove the handle with a handle puller.
Unscrew the packing nut from the faucet, using an adjustable wrench.
Unscrew the stem assembly from the faucet and remove it.
Unscrew the bib screw, which is the screw that attaches the washer to the faucet. Take the old washer off the faucet.
Unscrew the stem from the retaining nut and lift it out of the faucet. Inspect the thread on the stem; if it's damaged, replace the stem.
Remove the old O-ring from the stem and install a new one. Attach a new washer to the stem with a new stem screw.
Reassemble the faucet by reversing the steps you followed to disassemble it.
Turn the water back on at the main supply valve.