Look up the "breadknifing" technique. It is generally used to make edges straight, but you can also add a rocking motion to the breadknifing strokes. It is very easy to control the amount of curve.
When the curve is up to your specs, you need to build an entire new bevel on it. Gssixgun has written a great tutorial about that. One way to hone an eBay or damaged blade - Straight Razor Place Wiki

Bart.