Grass trimmers are power tools used in a similar fashion to lawn mowers. Grass trimmers cut smaller areas, such as close corners, edges or around hazards, that lawn mowers can't reach. Grass trimmers cut using a monofilament line rotating at high speed. The line wears down from the force of the cut and is reduced in length over time. Because of this wear, the grass trimmer will run out of line and need to be restrung.
Disconnect the trimmer from it's power source, whether it's a power cord on electric models or a spark plug for gas powered models.
Set the trimmer on a flat surface with the trimmer's head facing up. Press down firmly on the safety tabs, if provided, to unlock the cover of the trimmer head. Turn the cover counter-clockwise to remove the cover. Set the cover aside.
Pull the trimmer spool straight up from the chamber. Unwind and remove any remaining line in the spool. Discard the old line.
Cut an arm's length piece of new line. Pull the line straight. Hold the end of the line and the end of a measuring tape with one hand. Measure 12 inches from the end of the line. Cut the line at the 12-inch mark. Return the remaining line to the packaging and store it.
Insert one end of the 12-inch line into the notch found on the side of the spool. Pull the line 1 inch into the spool to keep it from slipping out. Look for an arrow on the outer face of the spool; the direction the arrow points is the direction the line must be wound onto the trimmer spool. Wind the line evenly in rows around the spool, creating layers until the spool is 3/4 full.
Thread the free end of the line through the eyelet of the trimmer spool housing; this will be a small hole in the side of the housing. Pull the line 6 inches through the eyelet while holding the spool. Put the spool back into the housing. Line up the safety tabs with the cover. Screw the cover back on in a clockwise direction and tighten.
Depress the center button of the trimmer head while pulling on the line. Advance the line a minimum of 1 inch. Remove the spool and reinsert the spool, if the line does not advance. Continue this process until the line can be advanced.