Some string trimmers use chains or ropes to cut weeds, most still use monofilament line made of nylon. String trimmers require the operator to occasionally re-load new line into the trimmer. This process is made simpler with the purchase of a pre-wound spool of line.
Read your instruction manual for your string trimmer to become familiar with the parts of your string trimmer as well as all safety precautions. Most all string trimmers have some kind of head that turns on a hub at one end of the trimmer and a motor on the other. The two are connected by a rod with a handle. The head typically houses string on a spool, and has a release button of some kind on the tip of the hub that releases more string once it has been tapped.
Disconnect your string trimmer from a power source. Gas powered weed trimmers require disconnecting a spark plug by pulling the boot off of the end of the spark plug. If you own an electric trimmer, disconnect the battery or unplug the cord.
Place one hand around the edge of the turning head to prevent it from spinning. Grasp the release button with the other hand and pull it straight out to remove it.
Pull the old spool from inside your hub and discard it. Clean the hub and the release button and inspect them for damages.
Pull the new spool from its packaging and inspect it for damage before installing.
Unwind 5 inches of monofilament line from the spool and place it in the notch on the side of the spool.
Insert the line in the exit hole in the hub.
Push the spool onto the hub until it snaps in place.
Pull the line extending from the hub to release it from the notch.
Push the release knob back onto the end of the hub.
Replace the spark plug boot on the back of the spark plug.