Double-string trimmers use the same type and length of line as their single-string cousins, except that it is cut in two halves. Both styles tend to eat up a lot of line, so it is good to have plenty on hand.
Hold the trimmer upside down to work on the trimmer head, or put the trimmer on a flat workbench.
Take off the spool retainer's cover. Grab the outer rim of the spool retainer; twist the center knob according to the directional arrow on the spool retainer. Pull off the cover.
Remove the inner reel and spring from the spool retainer. Separate the spring from the inner reel. Wipe these parts clean with the rag and inspect for any damage. Replace if bent, broken or heavily worn.
Cut off about 25 feet of new trimmer string (refer to owner's manual for specific lengths). Fold the string in half and cut it again.
Thread one end into one of the two holes on the inner reel. Pull the string all the way through until about 3 inches remains sticking out.
Fold the remaining 3 inches and wind the string around the string and inner reel. Use tight, even rows and make sure the layers don't overlap each other.
Leave about 8 inches out of the reel. Stick it into the notch on the inner reel to hold it in place. Repeat the same process for the second string.
Insert the two string ends into the eyelets on the rim of the spool retainer. Push the spring back in place inside the inner reel. Push both of them into the spool retainer. Secure them in place with the cover and lock it in place by turning it the opposite way.