If the drywall or plaster is damaged cut a larger hole and install a box that attaches directly to a stud or joist.
How to install a light switch in a finished wall.
Gently bend the wires and push the switch into the wall box.
Install a piece of the 12 3 with ground romex between the original switch location laying it in the notches you made in the studs running it up through wall to the second switches location.
White to white black to black and copper to copper.
Select boxes that meet local codes.
Twist a plastic wire cap onto each connection.
Then splice the fixture s black lead into the new black wire.
Install the wall plate.
For a ceiling fan or a heavy light fixture buy a fixture box that attaches to a fan rated brace.
When you need to add an outlet or switch in an existing wall you need to use a special electrical box called an old work box an old work electrical box may be metal or plastic and is specially designed to be installed in an existing wall where it has no support other than the wall itself.
After removing the black wire to the fixture s lead splice it into the new white wire.
Fold the wires into the junction box then place the fixture over the studs or nipple then attach with the cover plate per the manufacturer s instructions.
Place switch in the wall box.
Connect the white neutral wires with a wire connector.
To wire the ceiling fixture to the new wall switch first connect the copper grounding wire to the fixture.
Connect the wires as shown in figure a.
Install globes and bulbs.
Complete the project by mounting the light fixture screwing the switch to the box and installing the cover plate.
Tighten the mounting screws to secure the switch to the wall box.
Mark this wire black.
Cover with a switch plate.
To ensure that frequently used switches last longer purchase a spec rated or commercial device.
Select boxes that meet local codes.