I'm making a wedding cake. The cake will be a 4 tier round and they want scrolls all on the sides of the cake. What is the best method for making them? Should I use an impression mat then trace them with royal icing. If the cake is made with buttercream what should I pipe with? And if I use fondant would should I pipe with thats easiest?

On a buttercream cake, I pipe with buttercream. On fondant, I pipe with royal icing. It just seems to make sense to me.

As for the scrolls, I would find a photo of scrolls you like, then just free-hand copy them. It's easier than you think it will be. Surprisingly, they are easier and better looking if you pipe fast, rather than slow. Practice on the sides of a pan first, too. That will help.