mark the half way point and quarter way points on both the center quarter circle and the matching circle on the PP section, match and pin on the marks then when sewing, ease in the excess. Be sure to pin the ends of the curves as well. You'll get the hang of it after doing a couple. Remember that the seamlines have to come out even, there will be a slight ruffle in the seam allowance of the quarter circle but that will ease in when you press it after stitching.
Edited...to feel a little better, lightly mark in the seam line on both the pieces then lay one over the other matching the seam lines you marked and you;'ll see that they fit together....really!