Purchase a spool of mono-filament line for string trimmers. There are a variety of sizes available. Gasoline models are more powerful and use bigger lines from .080 inch to .155 inch. The thicker the line, the stronger it will be.
Lay the trimmer on the ground with the line head up. If your string trimmer is an electric model, unplug it.
Unscrew the cover in a counterclockwise direction. Some models may have safely tabs to unlatch. Depress the tabs with your index finger and thumb and then unscrew the cover.
Thread line an inch into the tab in the housing. Locate the arrow on the spool to determine the direction to wind on the line. Proceed winding the line around the spool in rows until the spool is almost full. Stop short of filling the spool to lessen the chances of the trimmer jamming.
Thread six inches of line into the eyelet in the housing. Insert the spool into the trimmer aligning the cover with any tabs on the housing. Screw the cover on.