How do I create straight diagonal lines on the side of a cake using butter cream frosting?

What I would do is mark the top and bottom edges of your cake with a toothpick size hole however far apart you want the lines. (i.e., 1", 2"). Then use a ruler to match up one point on the bottom to a diagonal point on the top and make a line with the ruler. Then just go over the lines with your buttercream icing.