Hold the rotating head at the bottom of your weed trimmer upwards so that you can easily reach the bottom.
Twist the bottom plate on the rotating head to the left to remove it.
Pull out the empty line spool. Use a rag to clean off the inside of the rotating head.
Remove the tape holding the line down inside your replacement spool. Untwist about 6 inches of line from the spool. If your model uses 2 lines, unspool 6 inches of line from each end. Feed the line through the hole(s) on the side of the rotating head.
Slip the spool down into place inside the rotating head. Replace the bottom plate by turning to the right until it is tight. Once it is in place pull the end(s) of the line slightly to remove any slack in the line.