Turn off power to the light circuit at the breaker box
Verify the power is off with a voltage tester
Remove the existing switch cover plate
Unscrew and pull the old switch out of the electrical box
Take a photo or label the existing wires
Disconnect the wires from the old switch
Check that the dimmer switch is compatible with the light type and wattage
Connect the dimmer wires to the corresponding house wires
Connect the ground wire to the ground screw or ground wire
Secure the wire connections with wire nuts
Carefully fold the wires back into the electrical box
Mount the dimmer switch into the box with screws
Attach the cover plate
Turn the power back on at the breaker
Test the dimmer switch and adjust as needed