Making a worthy copy of art work is one of photography’s most challenging tasks. You will find that pleasing the artist is next to impossible. This is because he/she will have both the original and your copy side-by-side. The reason this is arduous is because the photographic process optimizes skin tones and certain so called “memory” colors. Artist tends to demand more than our media can deliver. That being said, here is the recipe.
Make sure the camera and the art work are square with each other. Use a level and ruler to help position the camera. Use a moderate telephoto lens. This will force you elongate the camera to subject distance. This extra distance minimizes distortion that otherwise will result in an image that is other than a perfect rectangle.
Lighting must be uniform across the entire work or art. Lamps should be set at a 45° angle to the flat art work. These also should be backed away to help improve uniformity. You can uses bed sheets between lamp and subject to defuse. You might find that the best lighting is outdoor full shade on a sunny day.
Set the exposure so that the lens is operating about 2 f-stops below maximum aperture. Likely this will be your sharpest lens setting. Do use a tripod as this will improve sharpness and give you a better ability to duplicate your composition if you need to repeat the session.
Reflections from the art work are difficult to deal with. Use a polarizing filter on the camera. This filter must be rotated as you compose. The idea is to mitigate reflections as best you can. A professional setup will also have large polarizing filters over the lamps. It will help if you have a gray card. You can take your exposure readings, not from the art work but from a gray card positioned center on the work of art. After the reading, remove the gray card for the exposure. If you are sending out the work to be printed by a lab, each shot should be duplicated with a gray card centered on the work. This will assist the lab to make the necessary color balance adjustments. Again you are setting out to accomplish a challenging task and the learning curve is steep.