Sewing on a Curved Edge
Pin pieces together at even intervals around edges to be stitched.
Make light marks on the curved edge similar to the hours on a clock. (These are pivot points.) Begin sewing at one straight edge, removing each pin just before the needle reaches it.
At first mark on curved edge, sink needle down into fabric, raise presser foot, and adjust (pivot) the fabric along curve to maintain an even seam allowance.
Lower presser foot and continue sewing in short segments around perimeter of curved edge, pivoting fabric as in previous step, at each mark or as needed to maintain an even stitching line.
The result is a unit with uniform seam allowance around outer curved edge.