Turn off the main water shut-off valve in the home, typically in the basement or crawlspace.
Twist the shower control knob to drain any water in the pipe.
Pop off the plastic cap in the center of the shower control knob using a flat-blade screwdriver. If your knob or handle does not have a cover, skip this step.
Remove the screw in the center of the knob or handle to pull the part off the valve stem for the shower.
Unscrew the hexagonal stem nut holding the stem in place, using wrenches or a pair of pliers.
Pull the mixer valve straight out, removing the rubber washers ahead of and behind the valve, and setting them on paper towels in the precise order they are taken out of the valve socket.
Scrub the cylindrical mixer valve with an old toothbrush under hot, running water.
Wipe grime and grit from the washers with paper towels. If the paper towels are streaked with black flecks from the washers, it means the rubber is beginning to disintegrate. While you have the mixer valve disassembled, replace the rotting rubber washers with new washers of the same size. These pieces are available at hardware and home improvement stores.
Replace the washers and mixer valve in the reverse sequence from which they were removed.
Twist on the stem nut over the mixer valve and tighten with pliers.
Replace the shower knob or handle and secure with the screw.
Turn on the water at the main cut-off valve in the home.