Stop the engine by turning the power switch to the stop position. Remove the spark plug wire to ensure that the engine will not start accidentally.
Remove the spool retainer by turning clockwise and pulling it free from the string head. Keep the spring attached to the spool retainer and remove any old trimmer string.
Cut an 18-foot length of trimmer string with a utility knife. Locate the string's center and loop it around the hook in the middle of the spool, pulling the two ends of the string tight.
Wind the string in the direction indicated on the spool, ensuring that the line does not tangle.
Slide each end of the trimmer string into one of the two slots in the spool retainer.
Align the slots in the spool with the eyelets in the string head. Guide each of the trimmer string ends through its corresponding eyelet.
Install the spool retainer onto the string head by pushing down and turning clockwise. Pull on the spool retainer to ensure that it is secure on the string head.
Pull outward on the string ends at the same time to release them from the slots in the spool retainer and to rotate the spool retainer into the cutting position.
Push the spool retainer down while pulling on the string ends to ensure that the string will advance, and that the string head is assembled correctly.