Most trimmers are designed to work by rapidly whipping a plastic "string" around in a circle. Depending on the thickness of the string, it can cut through grass and even some large weeds, which is why trimmers are also sometimes called "weed whackers." Using a trimmer wears down the string, so it is important that you know how to replace it. Stringing a trimmer takes some practice, and you must purchase the proper thickness of string designed for your trimmer.
How to String a Trimmer
Remove the cap from the spool or cutting mechanism. A lock should be somewhere around or under the cap.
Press the locking mechanism to release the spool the string is wrapped around. Remove any remaining small pieces of string.
Cut a piece of your replacement string about 6 feet long. You do not have to cut it, but it makes it much easier to wind. Fold your string in half, and insert the folded end into one of the slots in the spool. These slots are sometimes called starter holes.
Wind the trimmer string around the spool in the direction shown by arrows or in the manual (usually counterclockwise). Leave 6 inches of each string hanging.
Snap one of the hanging strings into an upper notch. Bring the other string halfway around the spool and also snap it into an upper notch. Replace the spool in the housing (making sure it engages the locking mechanism, if you have one), and put the cap back on.