Remove a Knockout for a Metal Box
If you're , remove a knockout on the box for each cable that will enter the box. Use a screwdriver and hammer to break out each knockout (metal disc), then twist off the knockout with pliers.
Clamp Each Cable
Install a cable clamp for each cable, as needed. Standard plastic electrical boxes do not have knockouts and contain internal cable clamps. Metal boxes may have internal clamps; if yours does not, install a locknut-type clamp for each cable. Insert the threaded end of the clamp through a knockout hole and secure the clamp inside the box with the nut. Tighten the nut with pliers.
Mount the Box
Separate the circuit wires at the existing splice and loosen the cables as needed to make room for the new junction box. Mount the box to the framing (or other support structure) with screws driven through factory-made holes in the back or side of the box, as applicable.