Vivian,
From the directions, I would say like a regular seam. I'm not sure they have factored in a diagonal seam(it would use up more fabric). I would be more likely to do that if I was making this on my own...but in this case, I will not. Are you using a jelly roll? I am just using scrap strips and have yet to cut my background and medium fabrics. I could be up all night with nervous dogs (Az just got fireworks this year..so am expect to be up all night with canines! Too bad the instructions don't start today!) Happy New Year..and happy sewing!