A grass trimmer is designed for light-duty weed and grass cutting in tight locations or over terrain an ordinary lawn mower cannot reach. Unlike heavy-duty cutters that use saw-like blades for cutting, a grass trimmer uses a monofilament line. This attaches to a quickly-rotating head. If a monofilament line encounters a rigid surface, such as a stone or cement yard ornament, it will not cut these. Regular maintenance of grass trimmers requires that they be re-strung as the monofilament line wears out.
Cut a section of monofilament line from your spool that is between 15 and 28 feet long.
Unplug your string trimmer if it is electric, or disconnect the battery for a battery-powered trimmer. If your string trimmer is gas-powered, drain the gas and disconnect the sparkplug to prevent injury.
Turn the trimmer over and grasp the trimmer head in one hand. Place the palm of your other hand over the trimmer hub and wrap your fingers around the sides. Slowly rotate the hub in the opposite direction that it turns when in operation. The hub should unscrew from the head.
Lay the trimmer down and turn the hub over. You will see a spool inside attached to the axle of the hub. Grasp this hub in your free hand and pull it off the axle.
Unwind the remnant of monofilament line from the spool. The line should be attached in the center to a notch in the plastic spool. Remove the line.
Fold your new monofilament line in half and attach it to the spool at the notch along the fold.
Wind the double strands of line slowly around the spool in the direction indicated by the arrow printed on the spool. Be careful not to let the strands of the line cross.
Slip each of the ends of the monofilament line through an exit hole in the hub. Place the spool back over the axle.
Reattach the hub to the head of the trimmer by rotating back onto the trimmer in the direction that the trimmer rotates the hub when in use. Reattach the sparkplug and fill with gas, or plug in the battery or trimmer to operate again.