We decided to use a 12x12 box and make squares all over the walls. Then make a diagonal line through each box forming the zig-zags. (You can see the pencil marks in the pictures.) It was not easy! We kept getting the lines off by a few centimeters and I know they aren't completely straight. But, it's perfect to me!
I am just in love with it. Now that we have an idea of what we are doing we're going to do it again in her nursery. This time we're going to use a 6x6 because she wants slightly smaller chevrons.