1. Foundation paper is a guide. In my case, I've cut telephone book paper 8-1/2" square. Because I'm working from a tub of scraps with assorted fabric lengths, I can simply lay a fabric strip across the paper to check if it's long enough.
2. Because fabric strips are laid diagonally across the paper, the trimmed block edges are on the bias. The paper ensures those outside edges don't stretch. However, after removing the telephone book paper from the back of the block, I am treating the block very gently. Even so, I expect some blocks may be wavy. I will use a walking foot to join blocks to one another. (I also used a walking foot to sew strips to foundation paper.)