1 Prepare the line. 

The length and width of line you need varies based on your trimmer. If you purchase the wrong width of line, the trimmer will not work correctly, so don’t waste your money by simply guessing at the hardware store. If you are not sure what size line your trimmer uses, check online—the manufacturer’s website often has instructions, and if not the customer service department should be able to help you. The length to which the line should be cut also varies, from around 10’ to 25’If you are not sure, err on the side of too long; you can always cut it shorter later.

2 Make sure your trimmer’s engine is turned off. 

If it has a gearbox, make sure it is cooled down. This will help prevent accidents

3 Remove the retaining cap from the trimmer head. 

This will probably involve either unscrewing it, pressing one or multiple tabs, or a combination of the two. Some models use different mechanisms for removing the spool. These should be designed to be relatively intuitive, but contact your trimmer’s manufacturer if you are having trouble figuring it out.

4 Locate the starter hole in the spool. 

Insert the tip of the trimmer line, and wind in the direction of the arrows. Wrap the line in neat, straight rows, to prevent jams later on. When there are about 5” or 6” of line remaining, snap it into the retainer on the spool to hold it in place.

5 Line up the retainer with the slot on the outside of the head.

 Replace the spool into the trimmer head. Remove the line from the retainer, and pull it through the slot to make sure it is feeding smoothly. Reattach the retaining cap.