Unplug the trimmer's power cord from the electrical outlet.
Turn the trimmer over and depress the tabs on the side of the spool retainer, located on the very bottom of the trimmer head. Pull the spool retainer up and off the trimmer.
Remove the spool and then remove any remaining string.
Cut two pieces of string to approximately 9 feet in length.
Insert the end of one piece of string into one of the anchor holes in the spool. Wind the string around the spool in the direction indicated by the arrows on the spool. On the last pass, pass the string through the flange on the spool. Repeat for the other string.
Cut the strings sticking through the flange to approximately 6 inches in length.
Place the spool back onto the gear shaft.
Reattach the spool retainer by aligning it over the spool and then pressing it down until the tabs lock into position.