cut two pieces of film strip, one 25cm long and one 15cm long. (for a bow which is 9cm in lenght)

make a roll of each of the film strips, overlap the ends approximately 1cm. and sew across. (use wide stitches cause you'll be sewing along that seam some more)

squeeze the film in the middle so that the seam is in the middle and sew across again. repeat with the other roll.

sew the two film strips together, again, over the same seam.

put something in the middle of the bow and attach a hairclip or a pin in the back.
and you're done!