Disconnect the spark plug cable to prevent injury. Pull the cap off the spark plug on the back of the weed eater motor. Unplug an electric weed eater from the extension cord.
Turn over the weed eater to expose the spool seat. Twist the cap covering the spool to loosen it, following the arrow on the weed eater.
Pull out the spool carefully. Remove any old line left on the spool. Restring the spool with fresh line, following the directional arrows to wind it correctly.
Replace the spool; feed the nylon line into the outlet hole. Screw the cap back on tightly to ensure the spool stays in place.
Replace the spark plug.