I actually use the lines on my cutting board. (I've heard you're not supposed to do that, but my mat and my ruler agree, so that's how I do it) I would cut a 12.5" strip (12.5"x18) and two 3.5" strips (3.5x18), from the 18" side. Then cut a 12.5" square from the 12.5" strip and four 3.5" squares from each 3.5" strip. That's how I would do it.
) Ann