When the time has come to replace the line on your string trimmer, the first thing you to do is get the correct-size line for your make and model. Either refer to the owner's manual, or look at the string trimmer spool, which will always have line size printed or embossed on it. Always use the correct line as indicated, or you will compromise the efficiency and functional attributes of your brand of trimmer.
Remove the spool by either turning a center-locking nut, or pressing tabs in on the side of the spool housing and pulling it off. Every string trimmer manufacturer has its own method of spool removal, but you won't need special tools.
Cut an approximate 15-foot line for the trimmer spool, and if you have a dual feed trimmer, cut two lines to the same length, using scissors or garden shears.
Insert the new line or lines into a center hole on the string trimmer spool. Once inserted, bend the string sharply to create a "kink" that will hold it in place.
Wind the spring into the spool channel. Every manufacturer has directional arrows printed on the spool to show you the correct way to wind the line. Follow these arrows as a guide, and wind the line until the spool is full, then snap the line into the string-holding notches on the rim.
Snip the line off as it rests inside the holding notches, and allow approximately two inches of free-hanging line.
Replace the spool back into the spool housing. Pull the line out of the notches and thread each line through the corresponding eyelets. On some eyelets the line can just be pressed into place and a retaining ring can be snapped on to hold the line in place. On other spools, the line must be physically pushed through eyelet holes, much like threading a needle, and once in place, they can be pulled taut, locking them into place.
Secure the spool inside of the hub by either tightening down the center knob or snapping the tabs back into place.