Use a bench belt sander. There is no trick, or perfect shape you need to master. Some makers have a perfect radius, some like it a little flatter on the top radius. If your liking how the built lids are sounding take note of the wood, thickness, and radius.
From there its practice building and jigging if it easier for you.
You need to hold a consistent straight profile along the length, any low spots will cause headaches as you stroke the call.