You put a strip of wax paper around the cake the same width as the ribbon to prevent the fats in the frosting to go into the ribbon making it look wet and to prevent the ribbon from getting moisture in it and the color bleeding onto the cake. Then you attach the ribbon by just putting little pieces of tape on the wax paper. HTH!
That plastic wrap stuff? Hmmm, never used that. So, does it just attach to the back of the ribbon, or do you put it on the cake, like the waxed paper strip?
I did my first wedding cake last weekend & had to attach a ribbon around the base of my buttercream cake. I searched online and found a post/article on another caking site that suggested using clear packing tape on the back of your ribbon. It worked like a charm 
!! I rolled my ribbon out on a table, ran the packing tape down the backside of the ribbon and then trimmed off the excess with a knife. I used my #5 round tip to pipe a little excess icing around the base of the cake to attach the ribbon to.
I had no bleed through spots whatsoever! It was great & made the whole process go so easy in my eyes! 
I have always just wrapped the ribbon around the cake twice. The buttercream won't seep through to the second layer. I use a corsage pin on the back of the cake to hold the ribbon onto itself. So many different ways to do the same thing; I learn something new here everyday!
Well, I guess I am different. I wash my ribbon thoroughly and let it dry. Lightly rub crisco over the ribbon and wipe excess off. This will deepen the color of the ribbon a little, but will eliminate any grease spots because the ribbon is already greasy. I attach at the back with a pin like a corsage pin etc.
I have always just wrapped the ribbon around the cake twice. The buttercream won't seep through to the second layer. I use a corsage pin on the back of the cake to hold the ribbon onto itself. So many different ways to do the same thing; I learn something new here everyday!
Ditto!
I use a amall piece of double sided tape to attach ribbon to itself...never had any bleed through.
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