If the wires for your Christmas tree lights have a damaged portion, you may want to cut the wire to remove that portion and splice the good pieces of wire back together. Make sure you remove the entire damaged portion, or you will have to perform the process again. Always know where your plug is to ensure the lights are not plugged in while you are working on them, and store your lights neatly when not in use to prevent further damage to the wires.
Place a piece of masking tape on each wire on either side of the damaged portion you are cutting away, about 2 inches on either side of the damage. This will leave you with two pieces of tape on each wire strand--most Christmas lights have two strands, but some have three or more.
Label the two pieces of tape on one strand "A" with a permanent marker, then label both pieces of tape on the next strand "B," then continue the pattern for each additional strand. This will ensure you connect the proper strands back together in the splice.
Cut out the damaged portion of your Christmas tree light wires using wire cutters, taking away about an inch of wire on either side of the damage to ensure you are working with good wire for the splice, but leaving the portion you taped.
Select a crimp-on butt connector, also called a barrel connector, for each wire in the light strand. These connectors look like tubes with slightly larger ends than centers. They come in different sizes, so choose one that matches the gauge of your Christmas tree light wire, which will most likely be between 18 and 22 gauge.
Measure the thinner portion of the connector's barrel, then divide that length in half. Use wire strippers to remove that length of insulation from the end of the wire you will be splicing. This allows the thicker portion of the connector to house insulated wire while the active (thinner) part of the connector houses the bare wire, keeping the bare portion away from contact.
Place one end of one of your wires into one end of a connector, then place the connector and wire into the grooved side of your crimping tool. Squeeze the tool's handles together to crimp the connector shut.
Place the other wire with the same label in the other end of the connector. Place that end of the connector into the crimping tool's groove and squeeze the tool shut tightly.
Pull firmly on both ends of the wire to ensure they are secure in the crimp. If not, start the process over with a new connector. Once the connection is secure, tightly wrap the connection in electrical tape, stretching the tape as you wrap it around the wires. This will not hold the wires into the connector if the crimp is not secure, but it can reduce strain on the wires from movement and prevent debris from working its way into the crimp.
Repeat the crimping and taping process for each wire.