Couple points from experience (per the post Vigo linked to):
1) For measuring, use a wire, not a string. Tried with a string and I've found that it can result in an inaccurate curve due to the stretchiness of the string.
2) For cutting, the most accurate way is using a router jig. I tried a few inital panels using a jigsaw, but it was never as accurate as I wanted. With a router jig you're guaranteed a perfectly smooth curve.