Metal junction boxes have circular holes covered by metal tabs where wires and conduit may enter the box. The holes and tabs are formed by the metal stamping machine when the box is manufactured. These holes and tabs are called knockouts and, in order to make use of one, you have to remove the metal tab. Threaded adaptors with nuts allow you to attach conduit or clamp sheathed cable where it enters the box.
Place the screwdriver blade against the junction box knockout. The knockout is joined to the metal box in two places by a small tab. Place the screwdriver at the edge of the knockout away from the tabs.
Strike the butt of the screwdriver handle lightly with the hammer. The knockout will push out of the hole, but will still be attached.
Grasp the knockout with the pliers and twist it off. You may have to bend it back and forth a few times to remove it.
Remove the nut from the sheathed cable clamp. Insert the threaded end of the clamp into the knockout hole from the outside of the junction box. Screw the nut clockwise onto the clamp on the inside of the box.
Place the blade of the screwdriver against one of the notches on the nut. Use the blade of the screwdriver to push on the nut and tighten it clockwise. Tap the screwdriver handle lightly with the hammer to tighten the nut.
Some boxes have two or more sizes of knockout holes ranging in size from 1/2 inch diameter up to 1 1/2 inch diameter. Be sure you remove a knockout of the correct size for the clamp or other adaptor you intend to put in it.
Always turn off power to any box with live wires in it at the circuit breaker. Test to be sure the power is off with a no-contact voltage tester.