When you sew it together, you place the "piece of pie" on the bed of the machine and the open curved piece on top, having pinned them together at each end and in the middle. As you stitch you gradually match the top to the bottom piece. The two curves are really the same length! Don't try to pin all along the curve because that will only add to the difficulty. It's pretty easy once you relax and let the pieces match up as you stitch. :D