Here is a pic. The opening in this case was marked with a blue dye or marker. The gasket was aligned and the gasket opening was scribed on to the head (or intake) You can see the scribed circle, slightly larger than the cast port.
To the right you see the "extra material" being removed so the port is enlarged to "gasket matched" proportions.

Basically you are matching the ports from the intake, through the gasket, and then to the head. This will give a smooth port transition.