Do NOT do what I did - in my defense it was my first. I marked the light areas with the blue wash-away pen. I also marked the dark with the iron-off pounce. WHAT WAS I THINKING? When it came time to get rid of the lines, I had to get rid of the blue lines before I could iron off the white ones. What a disaster.
You could use rulers for both sides. That would eliminate most of the marking. Or you could mark with silver pencil It should show on both.