I'm attaching a few videos that I have found very helpful. They range from easy to complicated enough for a photography enthusiast to appreciate. Watching each one has given me a few gems that have immediately helped me capture especially my darker paintings.
The biggest change that made nearly all the difference for me is putting a pure white paper up to the painting before snapping the photo - pushing the button halfway to let the camera capture the white balance. How simple is that?! I hope this information helps you too.