Unscrew the welder plug's base from its cap with the correct type of screwdriver, usually a slotted screwdriver. Most replacement plugs use three screws, located between each prong, to hold the cap in place.
Push the welder's wire through the hole in the plug's cap. The part of the cap that tightens against the base should face the end of the wire. Some caps have wire clamps surrounding the hole. If so, loosen the wire clamp's screws with a slotted screwdriver before attempting to push the wire through.
Strip 1/2 inch from each wire with wire strippers.
Examine the welder plug's base. Wire retaining screws connect to wire terminals on the side opposite the prongs. The two side-by-side terminals connect to the 120-volt wires with silver and gold colored screws. The other terminal connects to the ground wire with a green screw.
Loosen the wire retaining screws with, depending on the plug's brand, either a Phillips-head or slotted screwdriver.
Wrap the stripped end of the green insulated wire around the green screw. Tighten the screw with the correct screwdriver.
Wrap the stripped end of the white insulated wire around the silver colored screw. Tighten the screw with the correct screwdriver.
Wrap the stripped end of the black insulated wire around the gold colored screw. Tighten the screw with the correct screwdriver.
Slide the plug's cap to its base. Align the base's screws with the cap's screw holes. Tighten the screws with the correct screwdriver.
Tighten the cap's wire clamp, if equipped, with a slotted screwdriver.