Gather your Christmas lights and find an area that's near an outlet. A power strip will allow you to test several strings of lights at once. The area also needs to be big enough to open the string of lights so you can see all the bulbs.
Unpack each strand of lights and untangle each string, being careful not to pull too hard on the wires or bulbs. Make a thorough inspection of each of the Christmas bulbs. Look for any missing, loose or broken lights and other damage to the wire or bulbs.
Replace any damaged, missing or loose lights.
Plug the string of lights into the outlet and leave them plugged in for several minutes to make sure they are working correctly.
Unplug the string of Christmas lights if they aren't working and check the plug area for the location of the fuse. Make sure there is only one fuse; some strings of lights have more. Examine each fuse and replace with new ones if required. Plug the lights back in to test again.
Test the individual lights on the string with a store-bought light tester and replace them as needed (see Resources).