A thin, flat package of heat sealed thermoplastic material is filled with plastic string trimmer line by inserting a mandrel into an opening in the rear wall of the package to receive the free end of a length of string trimmer line inserted through a slot in the periphery of the package to engage the mandrel. The mandrel is rotated while the package is held in a relatively fixed position to wind line onto the mandrel and, therefore, into the package, until the package is filled. The mandrel then is withdrawn, leaving the free end of the line accessible through the opening in the rear wall, and the other end of the line extends from the slot in the periphery of the package.