Lay the trimmer flat on a workbench. Locate the two tabs on each side of the trimmer cutting head.
Push both tabs in with the flat-head screwdriver and slide the spool retainer off. Slide the line spool out of the cutter head; remove any old string remaining on the spool.
Cut two 9-foot pieces of monofilament line. Insert the first piece of line through the anchor hole located on the upper part of the spool; wind the string around the upper part of the spool in a counterclockwise motion. Leave 6 inches of line sticking out after winding. Repeat this step with the second string, using the bottom part of the spool.
Slide the two pieces of line sticking out into the slots on the upper spool flange still connected to the bottom of the trimmer. Replace the plastic retainer, spool and spool retainer.