Turn off power at the breaker
Verify power is off using a non-contact voltage tester
Remove the switch cover plate
Remove the switch from the electrical box
Identify the line (hot) and load (switch leg) wires
Connect the line (hot) wire to the common terminal on the switch
Connect the load (switch leg) wire to the remaining brass terminal on the switch
Connect the ground wire to the green ground screw on the switch (or to the metal box if required)
If the switch has screws for both sides, connect the load wire to the other terminal and leave the line on the common terminal
If using a 3-way switch setup, identify travelers and common, then wire travelers to the two traveler terminals and common to the common terminal
If there are multiple wires in the box, use wire nuts or approved connectors for splices as required
Tuck wires neatly into the electrical box without pinching them
Reinstall the switch into the box and tighten mounting screws
Ensure switch is securely attached and not loose
Reinstall the cover plate
Turn power back on at the breaker
Test the switch operation (on/off)