AA friend of mine wants me to make the diagonals on a cake but on buttercream icing. Is this possible and how would I do it? I have the diagonal impression mat. Thanks in advance. :-)

you need to get a triangle with a 90 degree side. and place it on the cake and go to town. 

here is a cake central post, scroll down to see the tutorial.

http://cakecentral.com/t/679490/make-a-quilted-pattern

here is a static pic of the technique.