Curved Piecing

First of all I must say that I learned this method from Allison Aller in a tutorial on her old blog.

1) Cut the foundation for the back 2" extra the final size of your block (ex. 12"sq. for a 10"sq).
Choose the first patch for the centre and cut 5 curved sides as you wish and pin it. Fold the foundation back to not cut it then choose the second patch.

2) Put the second patch 1/4 at least under the first patch and with the cutter follow the line of the curve.
You will have the right curve.