Epson TM-300A Operator's Manual page 16

Fit the ribbon between the head unit and the ribbon mask. Then push the
cassette firmly into position.
Turn the ribbon-tightening knob five or six times in the direction of the arrow to
feed the ribbon smoothly into place between the head unit and the ribbon mask.
Check that the ribbon is not twisted or creased.
l
CAUTION:
Do not turn the ribbon-tightening knob in the reverse direction.
l