Place your trimmer on a flat surface with the head opening pointing upward. Disconnect the spark plug wire to prevent any accidents. Remove the bolt covering the head opening of your weed trimmer using a standard wrench and set it aside.
Insert the tip of the grease gun into the head opening or place the top over the head opening, depending on the sizes of the opening and the tip. Squeeze the trigger to fill the head with grease. Stop when a small amount of grease overflows the opening of the trimmer head.
Set aside the grease gun and wipe away the overflow of grease with a rag. Squeeze grease again into the head. The opening will overflow almost immediately; stop squeezing the grease gun trigger when it does.
Wipe away the excess grease again. Pull the string of the weed trimmer two to three times to position the grease without starting the machine. Fill the head with an additional small amount of grease and wipe away the excess.
Screw the head bolt back into place. Clean up the area and dispose of any greasy waste appropriately. Reinstall the spark plug wire and test out your weed trimmer.