I think you do it like any black strip method, but you need to cut the slashes in the same spots, and use a different fabric for each addition. So start with 12 different blocks, slash them all in the same spot, add the black, then add a different fabric to the other side of the black. Slash them all again in the same spot,then add the black, and again add a different fabric to the other side. Slash again... Wash, rinse, repeat. Does that make any sense? At least that is my best guess from looking at it. I bet it would be fun. I might try it for my toddler for Christmas anyhow.