Determine what you want to cut so you can install the correct string. The green Craftsman string cuts grass. The red string works for grass and small weeds and brushes. The black string works on large weeds and larger brushes.
Cut a 12-inch piece of string with a razor blade.
Locate the two sets of holes on the trimmer head. One set is labeled "Small." If you are using green or red string, you will use these holes. The other set of holes are labeled "Large." These holes are designated for the use of black string.
Place one end of your string into one of the holes appropriate for your size of string. Feed the string into the hole in the center of the cutter head and then feed the string through the positioning hole. Duplicate this process with the other end of the string.