I learned a very helpful hint from my Wilton instructor about making these roses. She told my class to use the Wilton premade white icing in the small can. It's the perfect stiff consistency to make flowers. Once I tried it, I never looked back. I could never get my buttercream smooth enough to make flowers. My roses always tore. Now I try to keep some of it around, just in case I need to make flowers for something. It colors really well so it's not a problem to use.
I recommend this stuff to anyone who wants to make handmade buttercream flowers for a cake.