How to I get a fondant ribbon to attach to a buttercream cake?

make a mixture of water and light corn syrup. this will be your "glue" for attaching things...gumpaste, fondant, etc. just dab or brush it on the back of your fondant ribbon and stick it to the cake. the water will eventually dry and the corn syrup will be left behind to keep things attached ashley vicos taught me this trick and i use it for everything now!

I just brush on water and it sticks like glue.

I assume you're going to put the bow on top of the cake? If so, then the two ladies above me have good ideas, just water should work, or water with some corn syrup.. or royal icing even.

if you're putting a bow on the side of a cake, i'd recommend using a support like a small dowel or a toothpick... and your glue should be something stronger like a thick royal icing

if you wrapping around the base i dont apply anything underneath the fondant ribbon.

I remember Indydebi once mentioned she uses a dollop of buttercream to attach pretty much everything on a cake.

Piping gel works beautifully.