Turn off the water supply for the bathtub. How this is done depends on how the plumbing in your house is set up. In newer homes, there is usually a cutoff valve for the bathtub located somewhere in the bathroom. Rotate the knobs clockwise to turn off the water supply for the tub. In older homes, you will have to locate the water utility box, usually near the street curb somewhere outside. These knobs are a bit harder to turn; use an adjustable wrench to turn the knob to the off position.
Remove the screw cover on the faucet handle by prying it off with a flat-head screwdriver. Remove the faucet screw with a Phillips screwdriver. Pull off the faucet handle.
Use the adjustable wrench to remove the packing nut. This is the first hexagonal nut you will see on the valve body. Turn counterclockwise to remove.
Remove the valve stem from the valve socket by turning counterclockwise with the adjustable wrench. Clean out any gunk or calcium buildup from the valve socket.
Wrap the threads of the replacement valve stem with plumber's tape. Insert the stem into the socket. Tighten with the wrench.
Replace the packing nut. Tighten with the wrench.
Slip the faucet knob over the end of the valve stem. Insert and tighten the faucet screw. Press the screw cover back into place.
Turn the water supply back on. Check for proper operation.