I dont think you need to hem them, sewing along the edge should be fine.
Mine are not even sewn but they are a solid ribbon not tartan, so that might be the difference. For Christmas I would be tempted to just cut them out so that I could wear them, and worry about sewing them later.

I found this on a listing on ebay:
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THE FLASHES ARE MOUNTED ON 1" (3CM) WIDE WHITE ELASTIC WITH VELCRO ADJUSTERS AND FIT ALL SIZES

THE BACK OF THE FLASH HAS A STRIP OF FUSING ON IT TO GIVE IT STIFFNESS TO STOP IT FLAPPING
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I Hope that helps.