String trimmers are as good as a goat when it comes to clearing weeds and grass--and a string trimmer won't jump the fence and eat your flowers. It will, however, regularly run out of nylon line, so replacing that line is a job you need to be comfortable doing yourself. This article talks primarily about the Weed Eater brand of string trimmers, although the general principles given here apply to all string trimmers.
Depress the locking tabs on the Weed Eater head. Pull off the head. Turn and pull off the locking ring and cover. Remove the spool.
Cut two equal lengths of nylon line. (Check your manual for the correct diameter and length of line.) Insert 1/2 inch of line into each of the two holes in the center of the spool. Wrap line in the direction of the arrows on the spool. Press the last 4 to 5 inches of line into the notches on the rim of the reel.
Replace the plastic cover, allowing the excess line to stick out the holes or grooves provided. Pull out any slack in the line. Replace the locking ring. Snap the head onto the end of the trimmer.