Push and turn the spool cover in a counter-clockwise motion to remove it.
Remove the lock ring if one is present on your weed eater. Unsnap it and pull it out. Take off the automatic feed button.
Wipe out the inside of the weed eater with a rag to remove any dirt, grass or other buildup.
Push down on the spool to release the tabs holding it in place. Remove it from its chamber.
Turn the spool over; take note of the arrow. Wrap the line of new string in the direction of the arrow. After it's wrapped around the spool, secure the string onto the spool by threading both ends through the respective holes at the top and bottom of the spool.
Reattach the spool cover and twist it clockwise until it clicks into place.