Is there a specific way I should attach a ribbon to the bottom of each tier?
I use jots bought in the scrapbooking section of any craft store. Little round double-sided sticky circles. Place a jot on one end of the ribbon, run ribbon round cake and secure ribbon on top of small jot at back of cake.
They can be purchased in different sizes. The smaller ones are great if you don't want much overlap of ribbon. If using small size, may want to put more than one of the little jots on the ribbon to be secure.
I make sure they do not touch my cake - only the ribbon.
I use a dot of melted chocolate or royal icing to attach the ribbon.