Epson TM-U300C Operator's Manual page 15

Fit the ribbon between the print 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 print 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
Close the printer
Ribbon mask