Grass trimmers use nylon thread to slice through grass and weeds. The thread is wound around a spool that is on the underside of the grass trimmer. The spool rotates about 6,000 times a minute. Over time, bits of the nylon thread wear down until there is none left on the spool. This means it is time to either replace the entire spool or buy new nylon thread to wind around the old spool. Rewinding your old spool may be cheaper than buying a new spool, depending on the model of your grass trimmer.
Flip your grass trimmer upside down and locate the spool underneath. The spool is usually held in place by a cap. You will be able to unscrew the cap itself, or you will need to remove the screw holding the cap in place.
Take the old spool off the grass trimmer and remove any residual nylon thread. Clean off any caked on dirt and grass by spraying the spool with a garden hose.
Cut approximately 18 feet of nylon thread. Fold it almost in half, making one side about 3 inches longer than the other side. Locate the arrow on the spool indicating which direction to wind the thread around the spool. Begin winding the thread tightly in that direction.
Continue winding until you have about 6 inches of thread left. Slide each end of thread into the holding notches on the outer part of the spool. Reattach the spool to the grass trimmer.
Locate the two small holes on opposite sides of the spool. Place one thread in each hole. Make sure that everything is tight and resume trimming.