just get a ruler and put the lines into a well crusted cake
That's a pretty little princess cake!
You can also use a scewer stick. Just mark your cake at the top every inch or however wide you want your diamonds, and then lay your stick at an angle down the sides of your cake.
Thanks! I used a medium sized square fondant cutter to make the design then went back over the lines with the back of a knife! It took forever, but I think it turned out ok.