I did a wedding a couple months ago with a square cake. I fought for almost 2 hours to get the ribbon to go up against the corners! How do you get them to go up o the corners and keep the corner look? I have a square cake soon with rhinestone ribbon and am stressing over the corners already!

Are the sides of your cake square, the only time I have issues is when the sides aren't completely square. 

I used melted candy melts the color of the cake to stick it to the corners.  

@ savannahquinn,

do you not find that the candy melts seep through and discolor the ribbon a little? Thanks :-)

Here are some suggestions from an old thread that might help

http://cakecentral.com/t/734097/fabric-ribbon-on-buttercream-cake