Unplug the trimmer.
Turn the trimmer upside-down and press in on the tabs on the trimmer head.
Lift the cap off of the trimmer head.
Lift the empty spool off of the trimmer head and clean grass and dirt from the spool and surrounding area.
Slide the new spool onto the boss, the part that spins the spool around. Use .065 inch round diameter nylon line.
Hook the end of the trimmer line into the eyelets on opposite sides of the spool so they extend out about 5-1/2 inches.
Line up the tabs on the spool cap with slots in the housing, and push the cap down until it clicks into position.