I made a Paddle Wheel quilt, the inside out log cabin. The outside of the 12 1/2 inch block is totally bias. The corners had a seam which need to go exactly into the corner when the sashing was on.
I laid my 12 1/2 ruler on my ironing board, drew around it with a pigma pen and made lines radiating out for the extension of the corners. I laid the block on that with the seams exactly in the corners and pinned them down Then I spray starched well, let it set a moment, and ironed it dry. My seams turned out perfectly into the corners.
I had muslin on my ironing board, but if you don't want to mark up your ironing board cover, lay freezer paper on it, mark on the freezer paper, and when you are finished it comes right off.