String trimmers, also commonly known as weed eaters or weed whackers, all use a type of nylon string to cut weeds. String size is always specific for each model of string trimmer, and you must know the size before you can begin string winding. An owner's manual will always tell you what size string you'll need, but if you don't have an owner's manual, string thickness is marked on the string trimmer spool itself. Using any other thickness of line other than what is required will result in poor or uneven line feeding characteristics and a loss of cutting performance.
Turn the string trimmer upside down, and remove the line spool. Spool removal varies from make to make; there may be a couple of side tabs to push in or a center plastic nut holding the spool on. Modern string trimmers are very user friendly, however, so you will need only your fingers and no special tools.
Get the correct size trimmer line by either looking at the actual spool (line size will be labeled on it), or looking in the owner's manual if available.
Locate the string anchor hole, which will be near the center of the spool. Insert the end of the string into the anchor hole; bend it sharply to keep it anchored in the hole. Once anchored, you are ready to begin winding string onto the spool itself.
Begin winding the line counterclockwise around the line spool, and as you lay the string down and begin to wrap it, you will see guiding directional arrows that have actually been embossed into the plastic. These arrows will show you precisely which way the line needs to be wound.
Wind the line accordingly by following the direction of the arrows. When the spool is fully wound, snap the line into a specifically made holding notch on the edge of the spool itself.
Cut the string off while it rests inside the holding notch, and allow about an inch or so of overhang. Use a heavy duty pair of scissors or shears to cut the line.
Grasp the line firmly between your fingers, and pull it out of the holding notch. While holding the line this way, place the spool back into the hub of the trimmer itself. Thread the line through the exit port hole in the hub, feed out approximately 6 inches of trimmer line and seat the spool into the hub by either snapping the tabs back into place or screwing down the center plastic nut.