Use water-proof ribbon from a florist, and use royal icing to glue the ends. I'd avoid any pins.
If you're using regular ribbonn, use was paper under it so that the BC doesn't leak through. If the frosting is fondant, any ribbon, without a lining, is fine.
I've never put ribbon around a buttercream iced cake. But around fondant, I used clear vanilla to attach the ribbon to the cake and to attach the ribbon at the ends, the vanilla usually works too. If not, I put tape, but in a way that it won't be seen. Since the ribbon is going to be removed anyway, so will the tape.
Good luck,
Alex
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