What is the best way to attach ribbon to cakes with buttercream icing? Would it be the same as fondant?

No, it's diffferent from fondant because your BC will make your ribbon greasy. I us 2 methods. If the ribbon is a dark ribbon I'll pre-grease it with shortening and then add it to the cake. If it's a lighter ribbon, I adhear packing tape (you can use contact paper too) to the back side of it prior to adding it to your cake.

I use meringue powder mixed with water to adhere ribbon to my cakes.

It works on both fondant and buttercream but with the BC I just stick the ribbon onto itself (at the ends)

HTH

how would you do this on a fondant cake? I tried everything and the ribbon wouldn't stick together. I even tried edible glue!