The subject should be uniformly lit, optimally by two sources of diffused light, each on the left and right side. The sources should be placed halfway between the camera and the image using reflector housings or some form of diffusers. The sources should be at the midpoint, and equal in intensity which can be tested by putting a pencil perpendicular to the work of art and seeing if the shadows cast are equal in darkness.
Now, use feedback again from an exposure meter or the camera to see if the light levels are the same. To use the camera to do this, use the telephoto setting and spot metering to concentrate the readings at a tight spot. Compare the readings at the center and at at the four corners.
Since the distance from the camera differs depending on the location on the image (closest at the center, more distant at the corners), use the minimum aperature of at least f/8.