I've done this and it was tuff. What I did, and someone may have better ideas, was I took a piece of thin glass to hold it flat. Stood it in a chair outside in the shade with no sun rays hitting the picture. Then set up my tripod and camera using no flash, Got my distance for the crop I needed and shot away. I also used my remote shutter cable and mirror lock up so I could get the best image out of this way of doing this. As another poster said it would be much better to have them scanned.
Allen