How do I scribe a curved sink?
Start by making a template. 

Using a dry-erase marker, draw a line on the sink where it will meet the new cabinet cutout. You can hold a ruler flat against the rectangular piece shown in the photo to guide the marker. 

Temporarily remove the currently installed rectangle and replace it with a rectangle of material from which the template will be cut. 

Cut a short length of scrap material (e.g. paint stirrer) to bridge the widest part of the gap, plus a quarter inch or so. Keeping this bridge strictly horizontal and flat against the rectangle, trace the marker line on the sink with one end. The other end will describe a path on the rectangle; mark this path with your carpenter's pencil. 

The line on the sink is to help you keep the end of the bridge in the plane of the rectangle surface, especially where the gap is widest. 

After cutting the template along the pencil line, match it against the line on the sink and trim it for a tight fit.