The best bit about doing this was using my new frixion pen - which disappears when you iron it. We actually got them for my older son who has terrible handwriting; his teacher suggested an erasable pen when they all moved up from pencils. But it turns out that the pen will also erase from heat - so I drew the diagonal lines on with a ruler, then quilted, then ironed and ta-dah! Vanished. It felt a bit naughty drawing on the quilt with a biro.
Some people don't like the pens because the marks can re-appear if you put the quilt in the freezer ... hmmmm I think there is a solution to that problem but I can't quite put my finger on it.