Turn off the trimmer and unplug it. Flip the trimmer over so that you are looking at the spool.
Hold the string head with one hand, and turn the spool clockwise with the other to release it. Pull the spool from the head.
Unwind any remaining string and remove it from the spool.
Cut two 9-foot-long pieces of replacement string.
Position the spool so that the bottom side, which will have spring sticking out, is facing up.
Insert the end of one of the two strings into the small anchor hole in the upper portion of the spool. Wind the string counterclockwise around the spool until there is about 6 inches of string left. Slide the string into one of the six notches that surround the spool.
Place the end of the second string into the small anchor hole in the bottom portion of the spool. Rotate this string counterclockwise as well until you have about 6 inches left and slide the end into the notch that is opposite of the one the first string is in.
Take the spool and flip it over so that the spring is facing down. Move the spool over the string head on your trimmer.
Thread each loose string through the eyelets on the inside of the string head. Lower the spool into the string head. When it is almost all the way in, pull hard on the strings to release them from the notches. Push the spool the rest of the way into the head.
Turn the spool counterclockwise to lock it into place.